Suitable verb form in the past tense sentence

Question

There's the next sentence:

"I saw Mr Brown ..... the building at two o'clock."

leave / to leave / having left

Which option would be the most appropriate to put in and why ? I really want to put in "was leaving", but there's no such option.

1. "to leave" - certainly no, there's no necessity in infinitive there.

2. "having left" - something strange, I don't know what is it.

3. 'leave' - may be 'saw' already indicates Past tense and we don't need to do that again. but why is there no ['s] ending for (he\she\it) ? is it acceptable to use bare infinitive with he\she\it without gerund or any modal verb.

Answer 1

To get a feel for this, ask yourself if you would use the past continuous or the past simple.

He was firing the gun. = five or six shots

I saw him (also: I heard him) firing the gun.

He fired the gun. = once

I saw him fire the gun.

He was crossing the road when the car hit him.

I saw him crossing the road, and I saw the car hit him.

We'll use the continuous form for things that take longer:

I saw him breaking into the car, and I also saw him driving away.

But for quick actions:

I saw him breaking into the car, but then I saw him stop.

If you don't know whether the action was completed, use the continuous:

I saw him climbing the mountain. Whether he got to the top or not, I don't know.

But we can also say this if you witnessed the whole event:

I saw him climb that mountain. I was with him every step of the way.

There's often not a lot of difference:

The 1930's saw fascism rise / rising throughout Europe.