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I remember I learned a structure like the one that this post’s title mentions:

All I {need to do | have to do | must do} is do something.

But is it correct to use "to do something" after the "is" — as compared with just plain "do something" without the "to"?

Or to be more specific, which of the following sentences in each pair is correct, or are both version correct? The second one in each case adds "to" to the formula:


Pair #1:

  1. All I must do is prepare myself for the test.

  2. All I must do is to prepare myself for the test.


Pair #2:

  1. What we should do is teach children how to think.
  2. What we should do is to teach children how to think.

As far as I could remember, both forms should be fine. But it seems that all the materials, texts, and articles I've read so far use the first form only.

Furthermore, I'd really appreciate it if you could show me some references, like webpages or some book, where I could find more information about this point.

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Related: english.stackexchange.com/q/3578 and bare-infinitive. –  tchrist Aug 12 '13 at 3:31
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I really think that saying something like “All I want to do is eat supper and go to sleep” without the extra to is the most normal way that is said. –  tchrist Aug 12 '13 at 3:40
    
@tchrist: Entirely agree. It is, though, easily confused with All I need now is to eat and sleep, where to is not optional. –  TimLymington Aug 12 '13 at 9:12
    
@TimLymington, sure it’s optional—removing it just changes the word class of ‘eat’ and ‘sleep’ from infinitives to pure nouns: “All I need now is eat and sleep”. (Okay, okay, ‘eat’ as a singular noun is perhaps not quite idiomatic …) –  Janus Bahs Jacquet Aug 12 '13 at 10:03
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marked as duplicate by tchrist, Kristina Lopez, TrevorD, p.s.w.g, MετάEd Aug 13 '13 at 0:42

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2 Answers

up vote 5 down vote accepted

Both are grammatically correct. The first is the one I'd use and is the idiomatic one. The second sounds fine and there's nothing wrong with it, but only less idiomatic and that's why you don't see it in your written materials. In addition to that, the to in there is unnecessary and doesn't really add anything to the overall meaning of the sentence. Most grammar books should cover these topics, but personally I am not aware of one. On top of that, that part of your question seems a bit off topic on this site. Good luck with your grammar.

Edit

Here's a link to Google Ngram that supports my assumption. Thanks to Peter for providing that.

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Asking for references is off-topic here? I don't think so. Do you have a reference for that? –  Peter Shor Aug 12 '13 at 3:08
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Here is support for your answer through Google Ngrams. The version with the infinitive is the traditional construction, but over the last century it has been replaced by the version without the infinitive. Fascinating! –  Peter Shor Aug 12 '13 at 3:14
    
@PeterShor Not references. He is asking for tutorials and books related to the study of grammar that covers this topic. Don't you think that's off topic? –  Noah Aug 12 '13 at 4:56
    
@PeterShor Thank you for the link. I'll embed that in my answer. –  Noah Aug 12 '13 at 4:57
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EDIT After searching the site (why didn't I do this before writing?) I found this answer to a similar question All you have to do is read” vs. “All you have to do is to read” Which explains far better than I could ever do.

Here are a few online references which deal with the aspect of choosing between the bare infinitive (or base verb) and the full infinitive form (with to).

My English pages

The bare infinitive

The bare infinitive is used as the main verb after the dummy auxiliary verb do, or most modal auxiliary verbs (such as will, can, or should..) Examples:

I do know him

I do like you.

I can do it .

In Wikipedia Uses of the infinitive

  • As complements of other verbs. The bare infinitive is used as complement of the dummy auxiliary do, most modal auxiliary verbs, verbs of perception such as see, watch and hear (after a direct object), and the verbs of permission or causation make, bid, let, and have (also after a direct object). The to-infinitive is used after many intransitive verbs such as want, aim, like, fail, etc., and as a second complement after a direct object in the case of verbs such as want, convince, aim, etc.

  • As a noun phrase, expressing its action or state in an abstract, general way, used as the subject of a clause or as a predicative expression: "To err is human"; "To know me is to love me". The bare infinitive can be used in such sentences as "What you should do is make a list." A common construction with the to-infinitive involves a dummy pronoun subject (it), with the infinitive phrase placed after the predicate: "It was nice to meet you."

Grammaring "Do/does/did bare + infinitive" and "Infinitive [after modals]"(http://www.grammaring.com/the-infinitive-after-modals)

Modal verbs

Modal verbs (can, could, may, might, will, shall, would, should and must) are followed by a bare infinitive:

I could hear the dog barking outside.
You must be joking.
He may have caught the train.
You should have told me earlier.

In the case of:

  • All I must do is to prepare myself for the test.
  • What we should do is to teach children how to think.

The to-infinitive is used after ought, used, be and have which are called semi-modal verbs or modal auxiliary verbs. They are different from other modal verbs in that they are followed by the to-infinitive.

Examples

  • She ought to go home now.

    You ought to see a dentist

    He used to go down the pub a lot.

    She never used to drink wine at lunchtime

    We have to wear a uniform for work (meaning of "external" obligation)

    I'm sorry, I have to leave now. (or "... I must leave now")

be + infinitive see the Wikipedia link

(often used to talk about a future arrangement or plan and for giving orders)

  • The President is to visit Great Britain in the Autumn.

    John is to leave tonight

    They are to marry next week.

    You are to do your homework

    Tell her she's not to be back later than midnight.

EDIT #2 / Conclusion

Both the bare infinitive and the to-infinitive forms are acceptable, with the first structure sounding more natural and clearly used more frequently in everyday speech. I was intrigued by the question and wanted to know why the to-infinite structure could be used. I believe in the sentence

All I must do is to prepare myself for the test

The is + to prepare expresses an arrangement or plan sometime in the future. This is grammatical, albeit very formal. It nevertheless sounds better than the equivalent:

All I have to do is to prepare myself for the test

The repetition of the particle, to, in have to and to prepare sounds awkward to my ears, but interestingly the meaning remains virtually unchanged. Have to and must both express a form of obligation, although grammar books will tend to emphasize that have to is more commonly preferred when speaking about the law, rules, and regulations; whereas must expresses personal choice (I must see that film) or necessity.

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