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I let him do it.

and

I allow him to do it.

Why does the latter require to?

What are the "rules" of using to with an infinitive? When is it necessary?

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In both OP's examples the verb do is an "infinitive". The basic rule is the infinitive is "marked" by to, but there are exceptions, notably...

1: Verbs of perception (feel, hear, notice, see, watch)
"I saw him do it"
2: let
"I let him do it"
3: make
"I made him do it"

If the primary verb isn't covered above (and isn't an auxiliary/modal such as can, may, must, shall, will) the infinitive requires to...

"I permit you to do it"
"I ask you to do it"
"I encourage you to do it"
etc., etc.

See here for more information on using the infinitive without to.


I just copied a list of verbs from the site I linked to, but it seems incomplete; "I helped him do it" is at least "valid", even if some might prefer to do. Plus there's "I had him do it" (where no-one would endorse the "marked infinitive" form).

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    The last examples you give are not exceptions: they're subordinate clauses with subjunctives; not infinitives. You can insert a that: “I suggest that he do that”. You can also use the infinitive (though it is rare and, to me at least, quite inelegant), in which case to is necessary: “I recommended him to do it”. (Note the subject in the subordinate clause being nominative, and the infinitive one oblique.) Jun 14, 2014 at 15:59
  • @Janus: You're quite right - I just got so carried away fannying about with that symbol I didn't have enough brain power left to think carefully about what I was actually saying. If I can't think of any other exceptions apart from auxiliary/modal/let/make (and if you can't help me out with any! :) I think I might have to revert that edit. Jun 14, 2014 at 16:24
  • “I had him do it” is the only one I can think of. Jun 14, 2014 at 16:28
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    @JanusBahsJacquet, Fumblefingers, Although it could be a subjunctive, it's easily as likely (in British English in any case) to just be a present tense verb I suggest he does that on Monday. Another proper exception, however, is help: I helped her do her homework Jun 14, 2014 at 16:33
  • @Janus: Your had and my help seem like enough to justify an edit. If that's all we can come up with I think the "There's a general principle, with a few exceptions as noted" approach still holds true. Jun 14, 2014 at 16:35

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