Возьми стул поудобнее, если мы будем долго смотреть фильм.

Возьми is the informal singular imperative of взять (to take).

So the speaker is talking to one person in a casual way, like a friend or family member.

• возьми = take! (to one person, informal)
• возьмите = take! (to several people, or to one person politely)

Russian often uses the perfective imperative here because the speaker means one complete action: take/get yourself a chair.

Russian usually omits subject pronouns when they are obvious from the verb form.

In возьми, the verb itself already shows that the speaker means you (informal singular), so adding ты is usually unnecessary.

• Возьми стул... = Take a chair...
• Ты возьми стул... is possible, but it adds emphasis or contrast.

Because стул is the direct object, so it is in the accusative case.

But for inanimate masculine singular nouns, the accusative looks exactly the same as the nominative.

So:

• nominative: стул
• accusative: стул

That is why you do not see a different ending here.

Compare:

• Я вижу стол = I see a table
• Я вижу брата = I see my brother

With animate masculine nouns, the accusative is different.

Поудобнее means more comfortable or a bit more comfortable.

It comes from:

• удобный = comfortable
• удобнее = more comfortable
• поудобнее = somewhat / a bit / preferably more comfortable

The prefix по- often makes the comparative sound softer, less absolute, or more like a practical suggestion:

• стул удобнее = a chair that is more comfortable
• стул поудобнее = a somewhat more comfortable chair / a nicer, more comfortable chair

In this sentence, it sounds natural and conversational.

In Russian, comparatives like удобнее, лучше, побольше, подешевле are very often placed after the noun in everyday speech.

So стул поудобнее is a very natural pattern, similar to:

• купить квартиру побольше = to buy a bigger apartment
• найти место получше = to find a better place
• выбери чашку поменьше = choose a smaller cup

This structure often means a noun that is more X.

Because Russian comparative forms like удобнее and поудобнее usually do not decline and do not agree like normal adjectives do.

A normal adjective changes:

• удобный стул
• удобная комната
• удобное кресло

But the comparative form stays the same:

• стул поудобнее
• комнату поудобнее
• кресло поудобнее

So поудобнее does not need a masculine accusative ending or any other agreement ending.

Because будем смотреть is the future imperfective, and it emphasizes the process or duration of watching.

Here the important idea is: we will be watching for a long time.

• будем смотреть фильм = we will be watching a film
• посмотрим фильм = we will watch the film / we will see the film (more as a complete event)

Since the sentence includes долго (for a long time), the imperfective смотреть is especially natural, because it focuses on the activity as something extended in time.

So:

• если мы будем долго смотреть фильм = if we are going to watch a film for a long time

This fits the idea much better than a perfective form.

That is a very common difference between English and Russian.

In Russian, after если, you can use future tense when talking about a future situation:

• Если мы будем долго смотреть фильм...
• literally: If we will be watching a film for a long time...

This is normal Russian grammar.

English usually avoids will after if, but Russian does not follow that rule. So learners should not try to copy the English pattern too closely here.

Долго is an adverb meaning for a long time.

It modifies будем смотреть:

• будем смотреть = will be watching
• будем долго смотреть = will be watching for a long time

So it explains how long the action lasts.

It is important for the logic of the sentence: if the film will take a long time, then it makes sense to choose a more comfortable chair.

Yes.

If you are speaking to:

• one friend / child / family member → возьми
• several people or one person politely → возьмите

So the polite/plural version would be:

Возьмите стул поудобнее, если мы будем долго смотреть фильм.

The rest of the sentence stays the same.

They are close in meaning, but not identical in style.

• стул поудобнее sounds more natural, conversational, and everyday
• более удобный стул sounds more formal, descriptive, or careful

In ordinary speech, Russian often prefers the comparative form:

• возьми стул поудобнее

rather than:

• возьми более удобный стул

Both are grammatical, but the version in your sentence sounds much more like normal spoken Russian.