In the first lesson on present tense verbs, it says for there are 3 types of verbs: -Ar, -Er and -R verbs. Under the -R verbs it says:
"All of them are short, though, consisting of only one syllable."
If there are only 3 categories, and -R verbs are only one syllable, then WHERE DOES förstår FIT IN!? Because it doesn't end in -Ar, -Er, or -R, unless -R verbs can be more than one syllable.
It's because it's formed from för + stå. This goes for lots of verbs that are formed similarly.
It's an exception that would normally be classified as an -R verb I guess. It isn't really different from other -R verbs besides having two syllables. It's conjugated the same way as stå is. Think of it as a modified stå, if that helps.