No, what would be grammatically correct is: I haven’t seen him since last Wednesday.
'Since the last ten years...' is correct, but is only part of a sentence.
"I said to him" is a grammatically correct sentence fragment since "said" is still waiting for its direct object. You said what to him? "I said "to him". would be a grammatically correct sentence.
No. Since "circumspect" is an adjective, "I appreciate your circumspect" is about as grammatically correct as "I appreciate your clean". Saying "I appreciate your circumspect behavior" is correct, similar to "I appreciate your clean appearance".
No. "I have known him for 5 years." Would be more correct.
No, it isn't correct. Since the word between is a preposition, the pronouns that follow it have to be object pronouns -- between you and me.
It has been 5 years since you knew him.Read more: It_has_been_5_years_since_you_knew_him._Is_this_sentence_grammatically_correct
The verb "led" is correct since it's the past tense form of "lead". As for "to a win", although grammatically correct, "to a victory" would be more fitting.
'Your teeth are' is the grammatically correct way of saying it, since 'are' is the 'is' for plural, and since the word 'teeth' is plural, it must be 'your teeth are'.
Yes, since the term "waitress" has not yet fallen into general disuse like the term "bartendress", for instance.
Yes it is.The meaning is ambiguous as it could be his childhood or my childhood (or, probably, both).
He was the author of some of the most classic Children's Books. Several of his books rhyme to the point where they don't exactly make sense, but since it is a form of poetry it is considered grammatically correct.
No, since Koreans is plural, but one is singular. Try "Koreans are the best drivers...." or "Koreans are among the best drivers...." or "Koreans are some of the best drivers..." If you said "A Korean is one of the best drivers.....", that would also be grammatically correct.