The general rule is: use the simplest form that works. In ordinary conversation about the past, that's nearly always simple past ("What did you say?", "I didn't know that", "I spoke to him just a few minutes ago/about an hour ago/earlier today/this morning").
The "why" is that you are talking about something that (a) started in the past, (b) ended in the past, (c) and doesn't have any implications for the present or future. It's a capsule, self-contained.
If you speak Russian you can almost always use the English form that corresponds to the Russian form you'd use. («Что Ты сказал?», «Я это не знал», «Я с ним разговорил только несколько минут тому назадь»). I don't speak any Slovak at all (tylko kilka słów po polsku) but it's quite possible that you can use Slovak the same way one can use Russian. It doesn't work for special features, of course, that exist in one language but not the other, but for the general case it's an easy way to understand whether you need to use a participial expression--which you mostly don't!.