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"I have never seen him drive the car before." <--why is "drive" not "drove"? How do you know? What is the rule for it?

"I have never seen him drove the car before." <--incorrect? If, so can you tell me what does this mean so i can understand why is this wrong.

I was busy reading what you have said. <--correct? why?
I was busy reading what you had said. <--correct? why?
I was busy reading what you have had said. <--correct? why?
I was busy reading what you had had said. <--correct? why?
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Hello, Jack and Johnno Emotion: smile

"I have never seen him drive the car before." <--why is "drive" not "drove"? How do you know? What is the rule for it?

Johnoo said the tense used in this sentence is the present perfect of "see" (have seen), which is true. That is the main verb of the sentence. Now, "drive" has a different subject (him); as Johnno said, it is an infinitive (a bare infinitive) so it does not show tense or aspect. The use of an infinitive in this case is not necessarily related to the tense of the main verb. In the following sentences, you would/could still use "drive":
"I saw him drive his car last night."
"I'll see him drive his car tonight."
"I should see him drive his car later."
"I made him drive his car into the garage."
"I won't ever let him drive my car."

Again, "drive" is the base form of the verb and has "infinitive" meaning, if that makes sense.
In your sentence, there is another form of "drive" that would make sense: "driving". This would make your sentence a bit more specific: your focus would be on the action itself, he was already driving his car by when you saw him.

"I have never seen him drove the car before." <--incorrect? If, so can you tell me what does this mean so i can understand why is this wrong.

Yes, this sentence is incorrect for the reasons posted above. Also, the subject "him" is an objective pronoun, it will not take a conjugated verb as predicate.

You could use "drove" in a sentence with similar meaning, but the form would be different:
"I saw (that) he drove this car into the garage."

1. I was busy reading what you have said. <--correct? why?
2. I was busy reading what you had said. <--correct? why?
3. I was busy reading what you have had said. <--correct? why?
4. I was busy reading what you had had said. <--correct? why?

Before dealing with the correctness or incorrectness of each complete sentence, there is something to be said about the verb tenses you used. In sentence #1 you used the present perfect, and in #2 the past perfect. Both tenses exist in English; what we'll see in a minute is if these tenses have been used correctly.
But in sentences #3 and #4 you have used verb combinations that don't exist in English. "Have had said" is not posible, nor is "had had said". So both sentences are incorrect.

Also, I guess we all agree that you used "say" here instead of "write" or "type", as many of us do when it comes to emails, online chat or message boards.

As to the sentences themselves, #3 and #4 are incorrect because the "tenses" used in them are not really possible or meaningful verb combinations in English.
In sentence #1, the present perfect is not the best choice. Think of it this way: you cannot possibly read something before it has been written. The action of writing is prior to your reading, so if you used a verb in the past for the second action (read), you indeed need a verb in the past too for the first (say).
I'd use the simple past in that sentence:
"I was busy reading what you said."

Sentence #2 is correct (I still prefer the simple past there, but there may be contexts in which the past tense will be a better choice). The meaning of your sentence is "Someone had written/wrote something before you got busy reading it."

Miriam
Comments  
Hi Jack, this is how I read it.

The first sentence is in the preesent perfect, the main verb is seen and is followed by the bare infinitive to drive. If drive were to be the main verb it would be in the past participle form 'driven', i.e. I have never driven a BMW. Also, 'before' is redundant as never indicates at no time in the past.

I was busy reading what you have said.

Incorrect as the present perfect must have some connection to the present and you are describing something in the past ( I was busy/I was reading). Your second sentence is correct as it relates apast action with another action in the past- past perfect.

The same goes for the next pair, the past perfect is correct, but it has the specific meaning that something has been said on your behalf and at your instigation. For example, getting a spokesperson to say something according to your wishes.

I'm sure Miriam will have something to add here(please be gentle MiriamEmotion: wink), but that's my two cents.

All the best, Johnno.
 miriam's reply was promoted to an answer.