Learn Polish: Marek i Ewa uczestniczyli w weselu i wznosili toasty. - Marek and Ewa participated in the wedding and raised toasts.

In Polish, if a plural subject includes at least one male person (here, Marek), the entire group is treated grammatically as masculine personal, also known as "virile" (męskoosobowy). Therefore, the past tense verbs must take the masculine personal -li ending. If the subjects were two women, like Ewa and Anna, the verbs would be uczestniczyły and wznosiły.

We usually talk about attending events using na (like być na weselu). Why does this sentence use w weselu?

It is dictated by the verb. While "to be at a wedding" is indeed być na weselu, the verb uczestniczyć (to participate) strictly requires the preposition w followed by the locative case. Therefore, "to participate in a wedding" must be uczestniczyć w weselu.

Why are the verbs uczestniczyli and wznosili in the imperfective aspect here?

The imperfective aspect is used here to describe ongoing background states and repeated actions. Uczestniczyli shows they were engaged in the event over a continuous period, and wznosili highlights that they raised toasts repeatedly throughout the party. If they had raised a single, completed toast, the perfective verb wznieśli would be used instead.