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I am trying to explain to someone why the sentence "I let him to sleep" is wrong, but I fail to come up with a good explanation other than "it's wrong". And now I am even doubting myself. Is this sentence even wrong in the first place? Or is it a correct sentence that makes sense? Shouldn't it just be "I let him sleep"?

Sadly, there isn't any more context than that, only this one sentence.

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    "I left him to sleep" is a more likely phrase. – Strawberry Aug 29 at 11:53
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It is incorrect simply because the idiom is "Let someone [bare infinitive]". In some situations, a "to-infinitive" is used, and in other the bare infinitive is correct. This is one of those times when only the bare infinitive is used.

Compare with the same structure "I made him sleep" or "I helped him sleep" (though in the last one the "to infinitive is also possible).

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    "I helped him to sleep" (suggested by your paranthetical comment) would commonly be understood as an elided "go". I.e. "I helped him [go] to sleep." – GalacticCowboy Aug 30 at 13:22
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    @GalacticCowboy I'm not sure about that. E.g. the similar "I helped him to walk" would not be understood as "I helped him go to walk." – Tavian Barnes Aug 30 at 16:02
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    "I helped him to sleep" can be interpreted in two subtly different ways. "I helped him to (engage in the activity of) sleep," where "to sleep" is the infinitive and the sentence is like "I helped him to walk." Or "I helped him to (achieve the state of) sleep," where "to sleep" is a prepositional phrase, a metaphorical navigation to a location, and the sentence is analogous to "I helped him to church." – kindall Aug 30 at 18:33
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Yes, I let him sleep is correct while I let him to sleep is incorrect.

Certain verbs take a bare infinitive, and "let" is one of them. Here's a link that discusses the issue more.

2

Well, it's not incorrect but means something entirely different from what you probably mean. "I let him to sleep" means that I performed the procedure of blood-letting on him until he reached sleep. "Let" in the meaning you surely intended does not take "to". It just doesn't: like with other words that are at the core of the language, it is comparatively useless to look for a reasoning since it has been in permanent active use since the times of old Anglosaxon and thus more resilient to following general changes of the language's logic than other elements.

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I find it helpful to think of constructions like "to [verb]" as if they were nouns. In this case "to sleep" behaves much like a noun. It could be the subject of a sentence: "To sleep is wonderful." It's very similar to "sleeping" which also acts as a noun e.g. "I like [sweets / the cat / sleeping]"

"I let him [noun]" just wouldn't be right, because you need to let someone do something - which means you need some kind of verb after "I let him ...". So "I let him [sleep]" is correct.

(I'm not saying this is the pure grammatical way of thinking about it - just that it might be a helpful way to think about these things without going into infinitives and gerunds and complicated terminology.)

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    But what about "I told him to sleep" or "I wanted him to sleep" -- these fit your reasoning but require the "to". – John Velonis Aug 30 at 15:24
  • @JohnVelonis Fair point, and decent counter examples. My way of thinking is just a rule of thumb can can be helpful in many cases, but definitely not all. A lot has to do with the other verb that you've got in the sentence. As often as possible, I like to have ways of thinking that don't depend on strict grammar rules - but as a first language speaker I suppose I can rely on my gut, but one does generally need rules when learning a second language. – Adam Reynolds Aug 30 at 19:30

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