Why do we use 'crawl' in: Fa saw the beetle crawl on her lap yesterday.?

Question

Why not crawled, crawling or was crawling

Answer 1

We use the present tense because it is in a clause.  

Replace the word with another word & see if it makes sense - run, jump, climb, etc.

It sounds correct because we are explaining the action that we observed.  When we observed the action, the beetle was, at that time crawling, so when we write it we use the present tense.  I also believe that crawling would be correct.  

Hope that helps!