conditionals?

I sometimes find myself wanting to make conditional statements, but it seems to be pretty complex in Japanese. As far as I know, there are a few versions:

ーえば　(with verbs and い adjectives), as in:
ドーナツは小さければ、二つ買って。 (If the doughnuts are small, buy two)
私は学校に行けば、死にます。 (If I go to school, I will die).

and I think with な-adjectives, it's like this?
火曜日は暇なら、ビールを飲みませんか？ (If tuesday is free, won't you drink beer with me?)

But I'm not sure where these constructions fall in certainty (relative to the English "if" and "when"). If the verb is in the non-past indicative, does it translate more accurately as "if" or "when"? Or is it all a function of context?
For example, what is the difference between: スーパーに行けば、牛乳を買います。 vs. スーパーに行くとき、牛乳を買います。