- I saw him going to city.
- I saw him go to city.
First one refers to gerund.
Second one refers to infinite.
Do they have the same meanings or different meanings?
The meanings are different.
In the sentence with the ing-form, the speaker saw only a part of the action of going to the city. He did not witness the person going from his starting point all the way to the city.
In the second sentence, however, the speaker witnessed the whole action. He saw the person leave home and also saw him doing the whole travel to the city. In this context this may be a bit weird, but here's an example where the difference is more easily explained:
- A. I saw him cross the street.
- B. I saw him crossing the street.
In the first sentence here, the speaker saw him move from one side of the street and make it across to the other side.
In the second sentence, however, the speaker saw him while he was already in the act of crossing the street or he saw him start, but did not see him finish crossing the street.
If that person crossing the street was hit by a car, sentence A would indicate that the speaker also saw the accident happen, while in B, the speaker may not have seen that person getting hit by a car.