Learn Hardcore Russian: После пикника мы убрали мусор и пошли домой. - After the picnic, we cleaned up the trash and went home.

Because после (meaning after) requires the genitive case: после + Genitive.
So пикник → пикника (genitive singular).
Same pattern: после урока, после работы, после фильма.

What case is пикника here, and how do I recognize it?

Пикника is genitive singular of пикник. For many masculine nouns, genitive singular often ends in -а / -я (though not always). Here it’s triggered by the preposition после.

Can I say после того как мы устроили пикник… instead?

Yes, but it changes the structure:

После пикника… = after the picnic (noun phrase)
После того как мы устроили пикник… = after we had/organized a picnic (clause)

Both are natural; the first is shorter and more common when the event can be named as a noun.

Why is мы included? Can it be omitted?

It can often be omitted: После пикника убрали мусор и пошли домой.
Russian frequently drops subject pronouns when the subject is clear from context. Мы is included for emphasis/clarity (for example, contrasting with someone else).

What tense and form is убрали?

Убрали is:

past tense
plural (because мы)
perfective aspect (completed action)
from the verb pair убирать (imperfective) / убрать (perfective)

Past plural in Russian typically ends in -ли: делали, сказали, пошли.

What’s the difference between убрали мусор and убирали мусор?

убрали мусор (perfective) = they cleaned up/removed the trash (finished result)
убирали мусор (imperfective) = they were cleaning up the trash (process), or used to clean up (habit)

In this sentence, the perfective убрали fits because it’s a completed step before going home.

Why is it мусор and not мусора?

Because мусор is the direct object of убрали, so it’s in the accusative case.
For inanimate masculine nouns, accusative = nominative, so it stays мусор (not мусора).
If it were animate masculine, accusative would match genitive (e.g., увидел брата).

Does убрать literally mean “to remove,” and how does it relate to “cleaning”?

Yes: убрать is broadly to remove / put away / tidy up. In everyday contexts, убрать мусор naturally means to clean up the trash (remove it from the area). It can also mean:

убрать комнату = tidy/clean the room
убрать со стола = clear the table

Why is it пошли and not шли?

Пошли is the past plural of пойти (perfective) and often means set off / went (started going) as a single completed move.
Шли is past plural of идти (imperfective) and emphasizes the process of walking: were walking / walked (as an ongoing action).

In a sequence like cleaned up and went home, Russian commonly uses perfective verbs: убрали + пошли.

What exactly does домой mean, and why isn’t there a preposition?

Домой means (to) home—it’s an adverb of direction. Russian often uses special direction words without a preposition:

домой = (go) home (toward home)
дома = at home
из дома = from home

So пошли домой is the standard way to say went home.

Is the word order fixed? Could I move things around?

It’s flexible. После пикника is placed first to set the time frame, but you could also say:

Мы после пикника убрали мусор и пошли домой.
Мы убрали мусор и после пикника пошли домой. (less natural; can sound like after the picnic modifies only the second verb)
Убрали мусор и пошли домой после пикника. (possible, but the ending после пикника gets extra emphasis)

Word order changes emphasis more than basic meaning.