Я не могу долго сидеть за компьютером.

Yes, Я can be omitted in many situations because могу already shows 1st person singular: I can / I am able.

So both of these are possible:

• Я не могу долго сидеть за компьютером.
• Не могу долго сидеть за компьютером.

Including Я makes the subject more explicit. It can add a little emphasis, contrast, or simply sound more complete depending on context.

Не могу is:

• не = not
• могу = I can / I am able

Могу is the 1st person singular form of мочь (to be able, can).

So grammatically:

• я могу = I can
• я не могу = I cannot / I’m not able to

This is present tense in form, but in Russian the present tense of мочь often corresponds to English can.

After мочь (can / be able to), Russian normally uses an infinitive to say what someone can or cannot do.

So the pattern is:

• могу читать = I can read
• не могу работать = I can’t work
• не могу сидеть = I can’t sit

Here:

• не могу = I can’t
• сидеть = to sit

Together: I can’t sit...

Russian often uses сидеть when talking about being seated somewhere, especially for a period of time.

So сидеть за компьютером literally means to sit at a computer, but in natural usage it often implies to spend time at the computer.

Using быть would be too general: it means to be, not specifically to be sitting.
Using находиться would sound formal and is not natural here.

So сидеть is the normal everyday choice.

Долго means for a long time or long.

It modifies сидеть, so:

• сидеть долго = to sit for a long time
• не могу долго сидеть = I can’t sit for a long time

Russian adverbs like долго do not change form.

Yes, the word order can change.

In the given sentence:

• Я не могу долго сидеть за компьютером.

the adverb долго comes before the infinitive сидеть, which is very natural.

But Russian word order is flexible, so you may also hear:

• Я долго не могу сидеть за компьютером.
• Я не могу сидеть долго за компьютером.

These are not all equally neutral in every context, but they are understandable. The original version is a very standard, natural order.

This is a very common question.

In Russian:

• за компьютером means at the computer
• на компьютере usually means on the computer, in the sense of using a computer as a device/platform

Examples:

• сидеть за компьютером = to sit at the computer
• работать за компьютером = to work at the computer
• установить программу на компьютер = to install a program onto a computer
• делать что-то на компьютере = to do something on the computer

So with сидеть, Russian normally uses за because the image is of someone seated at a desk/computer.

Because after за in this meaning, Russian uses the instrumental case.

Base form:

• компьютер = nominative singular

After за meaning at / behind in this expression:

• за компьютером = instrumental singular

So the ending changes:

• компьютер → компьютером

This is a standard masculine noun pattern in the singular instrumental.

No. За can take different cases depending on meaning.

Two very common patterns are:

• за + instrumental = behind / at
• за + accusative = to behind / for / after in some contexts

Examples:

• сидеть за столом = to sit at the table → instrumental
• идти за стол = to go to the table / take a seat at the table → accusative in some movement contexts

In your sentence, за компьютером is not about movement toward something. It describes location/position, so the instrumental is used.

Here it most naturally means I’m unable to or I can’t manage to.

It usually does not mean lack of skill. For skill, Russian often uses уметь:

• Я не умею плавать = I don’t know how to swim

But:

• Я не могу долго сидеть за компьютером means something like
  I can’t sit at the computer for long
  because of discomfort, tiredness, health, circumstances, etc.

It also does not usually sound like I’m not allowed to in this sentence. That meaning is possible with мочь in some contexts, but not the most natural interpretation here.

Сидеть is imperfective, and that fits well here because the sentence talks about a general ability/inability over duration.

• сидеть = to sit, with focus on the ongoing activity
• посидеть = to sit for a while, more bounded and perfective

In this sentence, долго and the general idea of sustained activity strongly favor сидеть:

• Я не могу долго сидеть за компьютером. = natural

Using посидеть here would sound less natural because посидеть usually means sit for a bit / spend some time sitting, not a general ongoing activity over long duration.

It can mean both, depending on context.

Literally, it is about sitting at the computer. But in normal speech, сидеть за компьютером often also implies spending time using the computer.

So the sentence may suggest:

• physical discomfort from sitting
• eye strain
• tiredness from computer use
• inability to work at a computer for long

Russian often allows this kind of slightly broader meaning through the physical verb сидеть.

Yes, if the speaker specifically means work at the computer.

Compare:

• сидеть за компьютером = sit / spend time at the computer
• работать за компьютером = work at the computer

So the original sentence is broader. It does not necessarily say the person is working; they might be studying, browsing, gaming, or doing anything else on the computer.

The main stresses are:

• Я
• не могу́
• до́лго
• сиде́ть
• за компьюте́ром

In particular, learners often need to notice:

• могу́ — stress on the last syllable
• сиде́ть — stress on the last syllable
• компьюте́ром — stress on те

Stress is important in Russian because it can move and is not always predictable.