Hi,

For anything happening now (the present tense) you can use do or does. They do eat; he does eat. You can also use is doing. He is doing homework.

For anything that happened in the past (the past tense), use did. He did his homework yesterday. They did not eat enough last night.

If something began in the past but is still continuing right now (the present perfect tense), use the have form of the verb. I have been logged into BeatTheGmat this morning for 20 minutes. (It started 20 minutes ago and I'm still here.)

If something began in the past and ended before something else that happened in the past (the past perfect tense), use the had form of the verb. I had just logged onto BeatTheGmat when my coffee machine beeped. I completed my log on first, and then the macahine beeped. Both events are over now.

If something is contrary to face, use "if were, [then] would". If the GMAT were not so tricky, people would not study as hard.

Cheers,