は is an auxiliary word to prompt the theme. Japanese sentences can generally be divided into two parts: topic and predicate. When speaking, we first highlight the central issue or scope. This is the theme, which is generally expressed by the auxiliary words は and も. Because the theme should be someone or something that can be understood at the mention, interrogative words cannot be used as the theme, such as:

This is a personal book.

Like a long nose.

どなた, Mr. Li?

が is a nominative auxiliary word, indicating the subject, that is, the object of the whole sentence, not the theme. が The sentence with the subject focuses on the subject. When an objectively described thing becomes the subject of a sentence, the subject is followed by the case particle が. For example:

どこがぁなたたちのですか.

ここがたちのです.

Who can do that? I don't want to do this.

Do it in private

どれがさんのですか in Tanaka.

Tanaka's それがさんのです.

In the sentence you gave, you used the auxiliary word は to emphasize what Mr. Zhongshan did, that is, this thing was given to me by Mr. Zhongshan in the car. If the prompt auxiliary word は is changed to the nominative auxiliary word が, the emphasis is on Mr. Zhongshan, which means that Mr. Zhongshan gave this thing to me by car, not others. I hope it can help you solve the problem.