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Quote from grammar book:-

By the end of the first year a baby will have already acquired some social skills.

Why is a comma not used before "a baby"?

In terms of movement, an infant will be able to reach a sitting position unassisted and pull himself up to stand.

Why is a comma used after movement?

Any help??

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    In both cases, because commas are not used according to grammatical rules. Commas are used according to phonetic rules -- if you can hear them, you write them. If you can't hear them, you don't. There is no necessary intonation dip after year, but there is a notable intonation dip after movement; thus the commas. When you read them, you should hear the dip like the author wants you to. Throw away any grammar book that tries to tell you grammar rules for commas; they don't have any idea what's really going on. – John Lawler Aug 23 '18 at 16:08
  • This question will likely get deleted for being a duplicate of numerous questions asking about the use of a comma after an introductory prepositional phrase, which commas are optional and so a matter of style, not grammar. – Billy Aug 23 '18 at 16:13
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Because grammar calls for a comma after an introductory prepositional phrase but does not require it. For extremely short prepositional phrases where a comma creates a pause that no one thinks or says, you probably shouldn't put a comma. The longer a prepositional phrase gets, the more recommendable a comma becomes. Still, all of that is based on one's subjective opinion and not clearly stated rules of grammar, making the comma after an introductory prepositional phrase a question of style, not grammar.

Here are some quotes from what various grammar sources have said about it:

A comma may also set off a single prepositional phrase at the beginning to make the sentence clear.

-http://englishplus.com/grammar/00000074.htm

Note the use of "may" in the above.

When a prepositional phrase expands to more than three words, say, or becomes connected to yet another prepositional phrase, the use of a comma will depend on the writer's sense of the rhythm and flow of the sentence.

-http://grammar.ccc.commnet.edu/Grammar/commas_intro.htm

Notice how it says above that the use of a comma will depend on the writer's sense, meaning it is subjective, a personal preference, not a hard-and-fast rule. Also, notice how it interjects "say," meaning "for example," so the aforementioned "three words" isn't actually a bona fide rule but only an example of the number of words in a prepositional phrase one might use as a rule of thumb for putting a comma afterwards.

When an introductory prepositional phrase is very short (less than four words), the comma is usually optional.

-https://www.grammarly.com/blog/commas-after-introductory-phrases/

Pat attention to how the above says "very short" and "usually" and doesn't actually say "is less than four words" but only includes "less than four words" as a parenthetical elaboration. All of that points to the employment of that comma as being optional.

All of that said, I'd have definitely put a comma after the introductory prepositional phrase "after the end of the first year." I think most grammarians would. But the writer can't actually be faulted grammatically for not putting one.

Also, do keep in mind that grammatical errors abound, even in books. Don't just assume just because you're reading it in a book or a magazine or some other publication, that it's necessarily right or necessarily what is even recommended.

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In both examples cited the sentence opens with two short introductory prepositional phrases in a row. When it is only one or two introductory phrases and both are short, the comma is optional unless the meaning of the sentence becomes unclear without the comma. (In other words, sometimes a comma must be inserted even after a very short introductory phrase to keep the meaning clear.)

Example: In all the classrooms contain 50 children. In all, the classrooms contain 50 children. (If you're following, you'll see that a comma is needed here because otherwise you start to read the meaning as "in all the classrooms," which is misleading.

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It's the introductory comma; it depends on style as well as on rhythm.

I don't think there's clear a rule as to whether the writer elides it or not; however, lots of texts leave out this type of comma.

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