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.
Best Answer
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).