Grammer Question, pls help

enter.

This is a catenative verb pattern. After see, hear, let, make, help, (and possibly a few other verbs), you can use the bare infinitive form, but not the past form.
saw the robber enter, heard a bird chirp, let the man sleep, make the children listen, helped me understand
CJ

And of course, it is correct to say:
Dennis saw the robber who entered his neighbor's house.
That changes the grammar to be a complete subordinate clause. The past tense is correct here.
The tone is a bit different. The infinitive form points more to the action ("enter"), and the clause form points more to the person ("robber").

[off topic]
Hey CalifJim: I've forgotten the detailed lingo used with English grammar and would like to brush up. Do you have a good reference, preferably on-line? Please post to my profile.... Thanks!

AlpheccaStarsDo you have a good reference, preferably on-line?

I would gladly pass such a reference along to you, but I'm not really familiar with the on-line sources. I have a small library of books I've collected over the years, but none of them treats the terminology as such. I'm sure you could find a useful reference or two through a Google search.
S

Hi Anon

Dennis saw the robber ( enter entered) his neighbour's house.

If the sentence is put in the Passive, it should be clearer why 'enter' should be used.

1 (a) The robber was seen to enter his neighbour's house. (Passive)

(b) Dennis saw the robber enter his neighbour's house. (Active)

2 (a) The robber was seen