(1) Original sense: learn how to ≠ learn to
One should make a distinction between the original sense of the structure learn how to and the common, colloquial, modern use of the structure learn how to. Originally, the expressions learn to do and learn how to do used to be quite distinct in meaning.
The form learn to do, then as today, used to refer to the acquisition of a skill.
- what is learned is an ability, capacity, power, behavior, etc.
- the knowledge is acquired procedurally, practically, by training, dynamically “learning by doing”
- a natural paraphrase is ‘acquire the capability of + gerund -ing’, like speaking
(1) he whyche in choppyng of Iohns head, had learned to behead good men, [...] did now lykewise lay handes vpon Iames thapostle
(from: Udall, Nicholas (1548) The first tome or volume of the Paraphrase of Erasmus vpon the Newe Testamente)
(= This describes the acquisition of “beheading” as a skill resulting from practical experience, not a theoretical understanding of “beheading” based on instruction.)
However, the original meaning of learn how to do, unlike today, was exclusively used to refer to theoretically learning about something.
- what was learned was the correct way of doing something, the structure of a concept, etc.
- the knowledge would be acquired theoretically, by explicit instruction, often from books, “learning by being taught”
- a natural paraphrase is ‘acquire good + deverbal noun’, like speech
(2) we must learne how to thinke of god as he is described to us in his word:
(from: Gouge, William (1637) A commentary upon the three first chapters of the first Epistle generall of St. Peter by Nicholas Byfield)
(= This describes the acquisition of the correct understanding of God based on instruction, not the mere ability to think about God resulting from training.)
Here are a few more examples to illustrate the original sense of learn how to:
(3) so if the youthe of Englande being apte of it selfe to shote, were taught and learned how to shote, the archers of england shuld not be only a great deale raker, [...] but also a good deale bygger and stronger archers then they be
(from: Ascham, Roger (1545) Toxophilus the schole of shootinge contayned in tvvo bookes)
(=The youth of England already have the ability to shoot (“being [naturally] apt by themselves to shoot”), but if they “learned how to shoot”, i.e. by teaching and instruction, they would benefit even more.)
(4) i learned how to become morally good; it is true i did; but is not the first precept in this knowledge, that the doctrine of manners, is it of self very vain? for it consists in doing, in action, not in knowing;
(from: Monmouth, Henry (1657) Politick discourses written in Italian by Paolo Paruta)
(= An explicit contrast is made between “learning how to be good” in theory, as “knowledge”, as opposed to its practical application “in action, not in knowing”.)
(5) Thou writest to men like thyself, subject to mortality; thou teachest them how to live, that they may learn how to die;
(from: Sheridan, Frances (1767) The History of Nourjahad)
(= This refers to the acquisition of “good death”, the correct way to die, so to speak, - in context, this means ‘learn how to accept death.’ It does not refer to the ability to die. In fact, nobody needs to learn that skill – it automatically happens to all of us eventually.)
The following graphic illustrates the original state of affairs:
Figure 1: Stage 1 - the original semantic distinction between learn how to and learn to
(2) General sense: learn how to ↔ learn to
At some point, the form learn how to has come to be used in contexts in which learn to do would originally have been used, i.e. in connection with learning an actual ability, skill or behavior. Hence, the two expressions are now essentially synonymous in many cases.
The following two examples illustrate the general sense of learn how to do something:
(6) You’re learning about Mesopotamia, true, but this assignment is teaching you much more. You’re learning how to analyze complex material quickly, how to pull essential ideas out of a text
(from: Rubin, Gretchen (2017) The Four Tendencies: The Indispensable Personality Profiles that Reveal How to Make your Life Better)
(= This refers to acquiring the skill of “analyzing” complex material. It does not (necessarily) refer to the correct way to conduct “an analysis” of complex material. Thus, “learning to analyze complex material” would mean the same thing.)
(7) But in truth, I didn’t want to spend my birthday learning how to ride a bike.
(from: Jamison, Leslie (2018) The Recovering: Intoxication and its Aftermath)
(= This refers to acquiring the skill of “riding a bike”. It does not (necessarily) refer to learning about “a bike ride”. Hence, “learning to ride a bike” would mean the same thing.)
The English language has thus changed. At some point, learn how to do was extended from purely academic, theoretical knowledge to, additionally, procedural, practical knowledge.
The following graphic illustrates the extension of general learn how to do.
Figure 2: Stage 2 - Extension of learn how to, resulting in synonymy with learn to
(2a) How did this extension happen?
One pathway for the semantic extension could be the inference that, if you know the manner, the way in which something is done, you are likely to also have the skill, know about the process of doing it (“learn how to” → “learn to”). So, English speakers may reason that if, say, you learn how to speak English, about the grammar, vocabulary and pronunciation, then you are very likely to acquire the actual skill of speaking English as well.
The inverse also works. If you have an ability, a skill, you may be quite likely to also know something about the topic from a more theoretical perspective (“learn to” → “learn how to”). As @FumbleFingers put it “ordinarily, in order to [verb], you need to know how to [verb].”
(2b) Both the original and the general sense are available in Present-Day English
It is important to realize that the original sense of how to do has not gone away. A new, general sense was merely added; it did not replace the original sense. The form learn how to do is now ambiguous between its original and its general senses. Thus, the two structures learn to do and learn how to do are not synonymous in all cases. There are some interpretations where an original learn how to do proposition is true but a learn to do proposition would be false.
The reason is that there can be a chasm between theoretical understanding and practical ability. There may be a discrepancy between knowing about a concept and actually applying the concept in practice.
Here are a few artificial examples where the difference becomes evident.
(8a) He is learning how to drive at school.
(8b) He is learning to drive in an old Chevy truck.
(=These two sentences can be interpreted as non-synonymous. You may learn about driving a car theoretically, (8a), but have not actually mastered the skill of driving a car, (8b).)
(9a) She learned how to relax from a therapist.
(9b) She still has not learned to relax.
(=You may have learned about a meditation technique to relieve stress, (9a), but have not actually figured out how to apply this technique, (9b).)
Example from @Jay
(10a) I want to learn how to diet
(10b) I need to learn to diet
(=You may want to acquire information about dieting, (10a), but you may not actually want to use the diet information or acquire more self-discipline, (10b).)
Here are two real-life examples of learn how to do from relatively modern texts that are probably intended in the original sense, so that it would actually be odd to use learn to do.
(11a) nothing gives them a better opportunity to learn how to behave in company
(from: Post, Emily (1922) Etiquette in Society in Business in Politics and at Home)
(=This obviously refers to learning about “socially acceptable, good behavior in company” because this is a book on etiquette in fine society.)
(11b) # nothing gives them a better opportunity to learn to behave in company
(= It is inappropriate to say “learn to behave” here because this is not about learning the “skill of behaving in company”, as when a mother rebukes her child so that it becomes well-behaved.)
(12a) Tomorrow’s schools must therefore teach not merely data, but ways to manipulate it. Students must learn how to discard old ideas, how and when to replace them.
(from: Toffler, Alvin (1970) Future Shock)
( = This obviously refers to acquiring a theoretical understanding of science. Students should learn about the conditions under which old ideas should be discarded.)
(12b) # Students must learn to discard old ideas.
(= It is inappropriate to say “learn to discard” in this context because this is not about convincing students to let their old ideas go no matter what. “Discarding” is not intended as an ability here.)
(3) General learn how to has become frequent recently
The general sense of learn how to has increased quite dramatically in frequency over the last two centuries.
The graph below shows the increase of the construction learn how to do, measured in occurrences per million words, in COHA, from 1820 till 2009.
Figure 3: The absolute frequency of learn how to in COHA, 1820-2009
As the graph shows, there has been a c. 16-fold increase in the absolute frequency of learn how to do over the period indicated.
The next graph shows the rise of learn how to do, measured as a percentage of all structure with help and a non-finite complement, in Google Ngram viewer, from 1800 to 2000.
Figure 4: Increase in relative frequency learn how to (vs. learn to) in Google Ngram Viwer
As shown, the relative frequency of learn how to do (vs. learn to do) has risen from c. 5% to over 25% over the indicated time period.
It does not seem implausible that this surge in “learn how to” is the reason why the present question has come up in the first place. It has only been quite recently that people could notice that learn how to and learn to have come to be used synonymously in many cases. At earlier stages of the language, speakers would have been much less likely to notice this particular aspect of language usage.
The graphic below sketches the increase in use of general learn how to do.
Figure 5: Stage 3 - Increase in the frequency of learn how to at the expense of learn to
(4) The acquisiton of new meanings of learn to do
Based on what is known about semantic change, one can predict with a fairly high degree of certainty that the current state of affairs is unstable and will evolve further. The reason is that languages do not generally allow for prolonged periods of synonymy between two items or structures. Speaking anthropomorphically, languages “like” distinct meanings for distinct structures; they “hate” free variation. Thus, it seems quite likely that learn to do will shift its meaning in semantic space into a niche where it is protected from the encroachment of, and distinct from, learn how to do.
This latest stage in the evolution of learn (how) to do is summarized in the following graphic.
Figure 6: Stage 4 - learn to shifts its meaning thus remaining distinct from learn how to
To find out what is actually going to happen, we would have to wait a century or so and see. I would not be surprised if speakers varied widely in their opinions, intuitions and assumptions about meaning differences between “learn to” and “learn how to”. That’s because during transitional periods, usage is not stable or fixed, but somewhat subjective and in flux.
For example, @mattbrehmer hypothesized that for learn to do, “the object of learning would be [a] prerequisite”. @FumbleFingers says that “"how" before an infinitive verb form places more emphasis on the learning as a relatively complex (or at least, "non-intuitive") process” etc. All of those subtleties may be correct, or they may be false. We just don’t know yet. But the uncertainty and variety of opinions are quite plausibly indicative of a linguistic state in transition.
(4a) Difference in formality
One part of the questions is if learn how to do might merely be a less formal version of learn to do. That is one possible outcome of a shift in the meaning. However, what we typically find is that the old form becomes formal and the new form becomes colloquial. Hence, if anything, one should expect that learn to do would acquire formal, not informal, overtones.
This is in fact what other commentators have suggested:
“In informal speech many people say 'learn how to [verb]' when they really mean just 'learn to [verb]'”
“rather than learn to being less formal than learn how to, it ordinarily is the other way round. [...] those with how in them have an informal ring.”
(quotes from @Jim)
(4b) learn to do is shifting towards deontic readings
One meaning associated with learn to do that has been mentioned in the comments several times, albeit only implicitly, is one of deontic modality. Deontic readings are about what’s moral, right, good, proper, appropriate, socially permissible, etc. Hence, the constructions learn to do X may shift towards a meaning that can be paraphrased as ‘come to realize that it would be better to do X’. This corresponds to my intuitions as well.
For example, @FumbleFingers says, “the more recent [examples of learn to obey] invariably mean "he learned that he must obey" [N.B. deontic must]. Imho it's the meaning of the verb "to learn" that has shifted over time.”
To illustrate, consider the following examples from the previous comments:
Example from @JohnLawler
(13a) John learned (how) to speak French whenever Madame was in the room.
(= General reading: He learned the French language, grammar, pronunciation etc. whenever Madame was in the room. Perhaps Madame helped him, or was his teacher etc.)
(13b) John learned to speak French whenever Madame was in the room.
(= Potentially new, deontic reading: John came to realize that it would be better to speak French whenever Madame was in the room. Perhaps Madame was strict or would get upset if he didn’t speak French etc.)
Example from @FumbleFingers
(14a) I wish you'd learn (how) to be quiet in church!
(=General reading: He learned the practical skill, or acquired a theoretical understanding, of being quiet)
(14b) I wish you'd learn to be quiet in church!
(= Potentially new, deontic reading: You should come to realize that it would be better to be quiet in church. Here, @FumbleFingers submits that it would be “completely wrong to include the word "how" - obviously the child already knows how to avoid making a noise”)
This would also explain the recent rise in occurrence of learn to not, which makes poor sense with the general reading (how can you acquire a non-ability), but good sense with the new deontic reading (you can realize that it’s better not to do something). The earliest example of the relevant sense I found is from 1989.
(15) Wyczynski says.'' We must learn to not abuse our powers.''
(from: Echikson, William (1989) Christian Science Monitor 1989, 12-13)
Also, adverbs indicating effortlessness like just do not work well with the general reading (you don’t “just" acquire knowledge or a skill – it takes time) but with deontic readings (you can “just” accept something). Indeed, all examples I’ve found are from very recently. This may further substantiate a recent shift in the meaning of learn to do towards deontic readings.
(16) But you have to learn to just move on.
(from: Barboza, David (2003) Facing Huge Debt, Large Farm Co-op Is Closing Down NYT, 2003, 03-19)
(17) Natalie, 42, was self-conscious about the fact that her twins did not look like her in any way. People often commented on this [...] ‘Are they yours?’ [She] was used to that had learned to simply smile and say a proud, ‘Yes.’
(from: Weidman, Evelina & Ellen Glazer (2013) Having Your Baby Through Egg Donation)