Verbs of perception (sight, hearing, smell, taste, touch)

Verbs of perception are those verbs that describe our five human senses. But don't forget that there are more than five words because of all the synonyms. So watch, spot, observe can all be alternatives to express the sense of sight.

These verbs are exceptions, but relatively friendly ones: They allow you to use both a a gerund or an infinitive. So you can say

I saw him crossing the street.

but you can just as well say

I saw him cross the street.

That means they make it pretty hard for you to make a grammatical mistake when using them.

There is a very little difference in meaning, though. In the first example sentence above you say that you saw the process of crossing, but you don't say you saw the person arriving at the other side of the street.

This is different in the second sentence. Here you saw the complete process from beginning to end. However, this difference is only relevant in rare cases.

Police: So you saw the man cross the street?

Witness: No, not exactly. I saw him crossing first, but suddenly there was this huge lorry coming from the left...

Police: And the man?

Witness: Can you see the big red spot near the zebra crossing?