Learn Swedish: Vi har inte träffats på flera årtionden.

This is a special rule for time expressions! When you state how long an action has been happening in a positive sentence, you usually use i (for example, "we have been waiting for hours" would use i flera timmar). However, because this is a negative sentence (vi har inte), you must use på to express the duration of the time that hasn't seen the action.

Why does the verb träffats end in -s?

The -s makes this a reciprocal verb, meaning the action happens between two or more people doing it to "each other". The active verb att träffa means "to meet [someone]", but adding the -s creates att träffas, which means "to meet each other". Therefore, vi har inte träffats means "we haven't met [each other]".

How is the word årtionden formed?

It is a compound word made of år (year) and tionde (tenth). So ett årtionde literally translates to a "year-tenth", meaning a decade. Since it is an ett word that ends in a vowel, it takes an -n to form the plural årtionden.

Could we say Vi har träffats inte på flera årtionden?