После пикника мы соберём мусор в парке.

Because после requires the genitive case.

• пикник = nominative dictionary form
• после пикника = after the picnic

For masculine nouns like пикник, the genitive singular often ends in -а or -я, so:

• пикник → пикника

This is very common:

• после урока = after the lesson
• после работы = after work
• после фильма = after the movie

Соберём is the future tense of the perfective verb собрать.

Here the idea is we will collect it / we will finish collecting it, so Russian uses a perfective future form.

• собирать = imperfective, process/habit
• собрать = perfective, completed result

Compare:

• Мы собираем мусор. = We are collecting garbage / We collect garbage.
• Мы соберём мусор. = We will collect the garbage.

So соберём fits because the sentence refers to a future completed action after the picnic.

They are an aspect pair:

• собирать = imperfective
• собрать = perfective

Russian verbs often come in pairs like this. The imperfective focuses on the process, repetition, or general activity. The perfective focuses on a single completed action or its result.

In this sentence, the speaker means that after the picnic, the garbage will be collected, so the perfective собрать is used, giving the future form соберём.

Here мусор is the direct object, so it is in the accusative case. But for an inanimate masculine noun, the accusative singular usually looks exactly like the nominative singular.

So:

• nominative: мусор
• accusative: мусор

That is why there is no visible ending change.

Compare with another masculine inanimate noun:

• стол → вижу стол

But with animate masculine nouns, accusative often matches the genitive:

• кот → вижу кота

Because в парке means in the park as a location, and location after в usually takes the prepositional case.

• парк = park
• в парке = in the park

If you were talking about movement into the park, you would usually use the accusative:

• идти в парк = to go to the park

So:

• в парке = in the park
• в парк = into/to the park

In this sentence, the garbage is being collected in the park, so в парке is correct.

Yes, very often.

Russian verbs usually show the person and number clearly, so соберём already means we will collect. Because of that, мы is often unnecessary unless you want emphasis, contrast, or clarity.

So both are possible:

• После пикника соберём мусор в парке.
• После пикника мы соберём мусор в парке.

Using мы can sound a little more explicit, like emphasizing we rather than someone else.

No, Russian word order is quite flexible, although some orders sound more neutral than others.

The given sentence is a natural, neutral order:

• После пикника мы соберём мусор в парке.

But other orders are possible, depending on emphasis:

• Мы соберём мусор в парке после пикника.
• Мусор в парке мы соберём после пикника.
• В парке мы соберём мусор после пикника.

The basic meaning stays the same, but the focus shifts. Russian often moves words around to highlight new or important information.

It is a prepositional time expression, not a full clause.

• после = after
• пикника = of the picnic

Together they mean after the picnic.

There is no verb inside that phrase, so it is not like after we have a picnic. If you wanted a full clause, Russian would use something like:

• после того как мы устроим пикник... = after we have a picnic...

But in your sentence, Russian uses the simpler noun phrase после пикника.

Соберём is pronounced roughly sa-bye-RYOM.

Important points:

• the stress falls on -ём
• ё is always stressed
• ё sounds like yo

So соберём is different from a spelling with plain е. In many printed texts, Russians often write е instead of ё, so you may sometimes see соберем, but it is still understood and pronounced соберём.

That is a very common feature of written Russian.

The standard stress is:

• пОсле
• пикнИка
• мы соберЁм
• мУсор
• в пАрке

Stress matters in Russian because it affects pronunciation, and unstressed vowels often sound reduced. So learning the stressed syllable is very helpful from the beginning.