Saying "when" in Japanese

So, in my textbook, I'll learnign the uses for "when," and it's quite confusing... I've asked about it on a forum i go on, and I thought I got it, but now I'm not so sure...
I'd just like to see if I have this correct. So, i guess I'll make up so random example.

1. I will eat when I buy food.
2. I ate when I bought food.
3. I will buy food when I eat.
4. I bought food when I ate.

OK, now in Japanese...

１．食べ物を買った時、食べます。
２．食べ物を買った時、食べました。
３．食べる時、食べ物を買います。
４．食べる時、食べ物を買いました。

Is that correct?

The way my textbook explains it makes it very hard to understand ^^;; I saw it like this:

1. future; the 'when' part comes first in the sequences of events.
2. past; the 'when' part comes first in the sequences of events.
3. future; the 'when' part comes second in the sequences of events.
4. past; the 'when' part comes second in the sequences of events.

So, is this all good? There aren't examples given in the textbook... But I wanted to try one and see if they'd work all four ways. I don't know if it was a good example... The second two examples seem kind of weird... haha.

The examples in the book are:

チベットに行く時、ビザを取ります。
チベットに行く時、ビザを取りました。
中国に行った時、ウーロン茶を買います。
中国に行った時、ウーロン茶を買いました。

よろしくお願いします！