about two verbs in a sentence to describe the past

by pan
(shenzhen.guagndong.china)

I am confused about a problem. I often see some sentences like:

1.I began to notice her at other times.

2.I saw him run up to the banker.

3.I saw some tourists arrive at the zoo.

All of sentences above, talk about the past and each one has two verbs. What I'd like to know is why the second verb (notice, run up, arrive)should be in the infinitive Form?