Learn Russian: Вчера на лекции мы много смеялись. - Yesterday at the lecture we laughed a lot.

For reflexive verbs, the ending you use depends on the letter that comes right before it. If it's a consonant, you use -ся (like он смеялся). If it's a vowel, you use -сь. Because the past-tense plural ending for "we" is -ли (which ends in the vowel и), we add -сь to make смеялись.

Why is "to laugh" a reflexive verb here? Did we laugh at ourselves?

Not necessarily! In English, reflexive verbs usually mean doing something to yourself (like "I wash myself"). But in Russian, some verbs permanently have the -ся or -сь ending, even if the action isn't directed back at the subject. Смеяться (to laugh) is one of these naturally reflexive verbs.

Why do we use на instead of в for "at the lecture"?

In Russian, when you are talking about attending an event, an activity, or a public gathering (like a lecture, a concert, or work), you use the preposition на to mean "at" or "in". The preposition в is mostly saved for being inside physical enclosed spaces.

Why does лекции end in -ии? Doesn't the prepositional case usually end in -е?

Normally, yes! However, feminine nouns that end in -ия in their dictionary form (like лекция) belong to a special group. Instead of changing to -е, their ending changes to -ии in the prepositional case.