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We usually say:

Let me know.

I will let him know.

Could the following be considered grammatical?

I will let know him.

Let know the guy.

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What exactly are you trying to convey? What is the sentence expected to mean? – Kris Sep 30 '12 at 8:45
@Kris just want to know if it's grammatical. – Noah Sep 30 '12 at 8:50
@Kris: Actually someone said that the latter were also grammatical but not very common. – Noah Sep 30 '12 at 8:51
@Noah: Whoever told you that the latter two (*"I will let know him" and *"Let know the guy") was absolutely wrong. Barrie England's answer is absolutely correct: Ungrammatical because they violate the rules for that English verb. They're "uncommon" only because most non-native speakers don't make that particular error. I never read it or heard it in 40 years of teaching ESL/EFL. – user21497 Sep 30 '12 at 9:09
up vote 4 down vote accepted

The last two are ungrammatical because a sentence in which let is used with a non-finite verb such as know follows the pattern (Noun) + Verb + Noun + Verb.

Other verbs which cause a sentence to follow this pattern are make, see, hear, have (when it means force to), help, know, feel, notice, observe and watch.

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