When do you use "Did + 1st form" instead of "2nd form" [duplicate]

Possible Duplicate:
“did shoot” vs “shot”

I often notice such sentences as: "EEG did show tumors"(from this week House M.D.) Why not "EEG showed tumors"? Is that form used to emphasize something?

Your guess is correct. Normally a simple indicative statement like this doesn't use the word did:

The EEG showed tumors.

However, when you want to emphasize the fact that the result was positive, you can use the auxiliary did:

The EEG did show tumors.

This puts extra weight on the fact that the tumors really were found. If you listen closely, you'll probably also hear the actors putting extra emphasis on the word did in this construction.