Grammar question
My grammar book says,
Tom got into his car and drove away. You saw this.
You can say: I saw Tom get into his car and drive away.

I thought “get” and “ drive “ should be past tenses. Because I have learned other grammar that if someone said something to you, you can say
Ex. She said she wanted to get a new car. “Want” should be past tense?

Question

Grammar question
My grammar book says, 
Tom got into his car and drove away. You saw this.
You can say: I saw Tom get into his car and drive away.

I thought “get” and “ drive “ should be past tenses. Because I have learned other grammar that if someone said something to you, you can say 
Ex. She said she wanted to get a new car. “Want” should be past tense?

Answer

In “She said she wanted X”, “she said” and “she wanted” are two different clauses — note that they each have an subject, repeated even though it is the same person. We can optionally use “that” to introduce the subordinate clause.

In “she wanted to get a new car”, there is only one clause, which includes the infinitive phrase “to get X”. This infinitive phrase is not a clause and does not have a conjugated verb. You already know this, so keep it in mind as we look at your other sentence:

In “I saw Tom get into his car and drive away”, there is only one clause (“I saw Tom X”). “Get into his car” and “drive away” are bare-infinitive phrases that complete the meaning of the one and only clause. The work the same as the infinitive phrase in your other sentence (which you understood already).

Answer

"Get" cant be conjugated in the past tense because there is the verb "see" before it which is already in the past (grammar rule of two verbs in the same context)
And "drive" followed the tense of "get" due to conjunction