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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.
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).
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.
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.
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.)
