1

They must let him (to) make his own decision.

I participate in voting in social network and 60% voted for not using to. But I'm not sure if it's correct to use the infinitive here like we do with

They must allow him to make his own decision.

2

Let, along with make, have, and sense verbs like see, watch, hear, and feel, can take infinitive complement clauses without the complementizer to, but with a subject NP (like him).

  • They let him escape, but not *They let him to escape.
  • They saw him escape, but not *They saw him to escape.
  • They made him leave, but not *They made him to leave.

This is not true for most verbs. In particular, it's not true of allow, forbid, permit, or order -- verbs similar in meaning to let -- which require a to complementizer, and are ungrammatical without it:

  • They allowed him to escape, but not *They allowed him escape.
  • They forbid him to leave, but not *They forbid him leave.
  • They ordered him to leave, but not *They ordered him leave.

It's unusual not to allow the to complementizer, and it marks the verbs that do it as somehow special. Certainly those verbs are less lexical and more grammatical than others, having
started on their journey to auxiliary verb status by taking part in hundreds of idioms.

  • Probably mainly because of He maketh me to lie down in green pastures, I feel at some point in the past including to in your first two examples would have been more acceptable, if not actually standard. Is there any reason to suppose there's a general tendency for verbs lose complementizers in this way, or is it just something that sometimes happens, not part of an identifiable "trend" in English? – FumbleFingers Reinstate Monica Dec 7 '15 at 19:23
  • In my language this has different meanings. For example with to it invokes the indicative mood and without it the subjunctive mood. Does this apply to English? – Alejandro Dec 8 '15 at 13:53
  • @ale - No, "to" doesn't indicate anything about the mood in English. – stangdon Dec 8 '15 at 20:27
  • What if you removed it? – Alejandro Dec 8 '15 at 20:31
  • @Ale - I'm not sure I understand your question. Put simply, whether "to" is there or not indicates nothing about the mood in English. – stangdon Dec 8 '15 at 21:45
1

It is correct without the "to": They must let him make his own decision.

1

According to the Free Dictionary, the verb let:

usually takes an infinitive without to or an implied infinitive

(emphasis added).

The Cambridge Dictionary also has information about the distinction of allow, permit, or let. This also shows that let is used without to.

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