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The best thing to do is to concentrate on something else.

I've known in the sentence like that 'to' should be omitted. So, I wonder if the sentence is grammatical.

*The source of the information was some English test prep book. According to it, in case the subjective involves "do" and the verb is "be", the complement should be "infinitive" without 'to'.

Well, now I guess it doesn't need to omit 'to', though.

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What is the source of your information? Why did you consider credible? Always try to cite a reference. – Kris Feb 3 '13 at 10:23
Welcome to EL&U. Off topic (proofreading request). Please recast as a general question. Thanks. – MετάEd Feb 3 '13 at 14:18
There are two complementizer to's in your sentence. The first one (the one introducing the relative infinitive to do is not dispensible; it is required to mark the relative infinitive. The second one (the one introducing the infinitive to concentrate) is dispensible, since NP complementizers are normally only required when they begin the sentence; they're optional elsewhere. – John Lawler Feb 3 '13 at 17:42

It can be omitted, but there is no should about it.

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If we are to follow usual conventions of English grammar, the preposition "to" should not be omitted. The reason is as following ...

  1. He has {to go to school}.

    He has {going to school as a necessity}.

    He has {necessity of going to school}.

    i.e., He has {complex noun}.

  2. His responsibility is {to go to school}.

    His responsibility is {going to school}.

    i.e., His responsibility is {complex noun}.

By complex noun, I meant either

  • a simple noun with or without adjectival modifiers
    His responsibility is {a new school).
  • prepositional phrase
    His responsibility is {of the school)
  • relative wh-clause (what, who, where, etc
    His responsibility is {which girl he likes}
  • infinitive phrase
    His responsibility is {to go to school}
  • present-participle/gerund phrase
    His responsibility is {going to school)
  • coupled predicate
    His responsibility is {that going to school is enjoyable)
  • modified complex noun
    His responsibility is {largely that of going to school is enjoyable)

Therefore, saying

The best thing to do is {concentrate on something}

is akin to saying

His responsibility is {go to school}

which is a poorly written sentence because {go to school} does not fit as a complex noun phrase. It is a verb-phrase.

If you wish to drop the preposition, you would need to turn it into a gerund phrase

The best thing to do is {concentrating on something}

The phenomenon of dropping the preposition of an infinitive phrase is a misjudgment practiced by iphone/android-texting kids, and often overlooked by their grammatical guardians when the verb in the infinitive phrase is longer than two syllables.

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