simple grammar verification

Hi. A very basic question. I encounter this sentence in my textbook:

勉強が終わってから、一緒に食事しませんか。

What I'd like to know is, is it possible for "owatte" to be "owatta" or "owaru" instead? Are they grammatically wrong? Or are their meanings different? Thanks.

勉強が終わってから、一緒に食事しませんか。

The meaning would change if you changed the verb.

The original sentence says:
- Wouldn't you like to eat together with me after we're done with the studies?

If you changed to "owatta," then:
- Since I'm/we're done with the studies, wouldn't like to eat together with me?

If you changed to "owaru," then:
- Since I'm/we're about to get done with the studies, ....

If you say

勉強が終わったら、一緒に食事しませんか。

is it OK?

Original sentence:

勉強が終わってから、一緒に食事しませんか。(Here, "...kara" means "after (doing something)")
- Wouldn't you like to eat together with me after we're done with the studies?

Zyzzyva's two sentences:
勉強が終わったから、一緒に食事しませんか。(here "...kara" is more like "since/because...")
- Since I'm/we're done with the studies, wouldn't like to eat together with me?

勉強が終わるから、一緒に食事しませんか。(the same as above)
- Since I'm/we're about to get done with the studies, ...

PerezDeservedIt's sentence: yes, you can say that :)

勉強が終わったら、一緒に食事しませんか。(here. "..(ta)ra" means "if/when...")

- If/when/once our studies are done...