I saw this sentence in a book, and the verb "happen" has no "s":
I have seen it happen to friends.
May the sentence be wrong? and if it is correct in what cases we can not give "s" to a verb?
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The sentence is correct. Here you see the construction "to see somebody do something." Instead of "see" there can be "hear," "watch," or "listen," for example.
"I have seen it happen to friends" means the same as "I have seen how it happens to friends." Omit "how" and you've got the construction. Here are a few other examples:
Can I watch you/her/him/them play the guitar? - Yes, sure.
I hear you whisper but I don't understand what you are whispering.
Can I listen to you sing my favorite song? - No problem. I'll sing it for you.
Can I see your baby sleep?
Note that after the predicate you need to use the objective pronoun. So, it would be incorrect to say "Can I listen to he sing?" You need to say "Can I listen to him sing?"