Этот стул неудобный, поэтому я сижу на диване.

Because стул is a masculine singular noun, and этот must agree with it.

In Russian, demonstratives like этот change form to match the noun’s:

• gender
• number
• case

So in the nominative singular:

• этот = masculine
• эта = feminine
• это = neuter

Since стул is masculine, you say этот стул.

Неудобный is an adjective meaning uncomfortable or not convenient, and it agrees with стул.

Because стул is:

• masculine
• singular
• nominative

the adjective also takes the masculine singular nominative form:

• неудобный

So:

• этот стул неудобный

is literally this chair is uncomfortable.

Yes. Этот стул неудобен is also correct.

Russian often has two adjective forms in sentences like this:

• long form: неудобный
• short form: неудобен

Both can mean is uncomfortable, but they feel a little different:

• неудобный is very common and neutral in everyday speech
• неудобен is also correct, but can sound a bit more formal, concise, or bookish

So:

• Этот стул неудобный = very natural, conversational
• Этот стул неудобен = also fine, slightly more formal

Because the sentence contains two clauses:

• Этот стул неудобный
• поэтому я сижу на диване

The word поэтому means therefore, so, or that’s why, and it connects the result to the first clause. In Russian, a comma is normally used before it in this kind of sentence.

So the structure is:

• This chair is uncomfortable, so I’m sitting on the sofa.

This is a very common question.

• поэтому = therefore / so / that’s why
• потому что = because

They express the relationship from different directions.

With поэтому:

• reason first, result second
• Этот стул неудобный, поэтому я сижу на диване.
• This chair is uncomfortable, so I’m sitting on the sofa.

With потому что:

• result first, reason second
• Я сижу на диване, потому что этот стул неудобный.
• I’m sitting on the sofa because this chair is uncomfortable.

Both are natural; they just organize the information differently.

Because сижу comes from сидеть, which means to be sitting. It describes a state, not the action of moving into that position.

Compare:

• сидеть / сижу = to sit, to be sitting
• сесть / сяду = to sit down, to take a seat
• садиться / сажусь = to be sitting down, to seat oneself

In this sentence, the speaker is already on the sofa, so Russian uses сижу:

• я сижу на диване = I’m sitting on the sofa

If you wanted to say I’ll sit on the sofa, you could say:

• Я сяду на диван.

Notice that the case changes too: на диван instead of на диване.

Because на диване expresses location: where someone is sitting.

With на, Russian often uses:

• prepositional case for location
• accusative case for motion toward something

So:

• я сижу на диване = I am sitting on the sofa → location
• я сяду на диван = I will sit down onto the sofa → motion/destination

This is a very important Russian pattern.

Because after на in the meaning of location, диван goes into the prepositional case.

The base form is:

• диван

The prepositional singular form is:

• диване

So:

• на диване = on the sofa

This is a regular pattern for many masculine nouns:

• стол → на столе
• шкаф → в шкафу
• диван → на диване

Because in normal usage you sit on a sofa, not in it.

Russian often uses на with pieces of furniture that are treated as surfaces or seating places:

• на стуле = on a chair
• на диване = on a sofa
• на кресле is less common than в кресле, because an armchair is more enclosing

For диван, на диване is the normal choice.

В диване would usually sound strange unless you literally mean inside the sofa, which is not the intended meaning here.

Because Russian has no articles.

English says:

• this chair
• the sofa
• a chair

Russian usually leaves article-like meaning to context, word order, or words like этот.

So:

• Этот стул = this chair
• на диване could mean on the sofa or on a sofa, depending on context

In this sentence, the meaning is clear without articles.

No, Russian word order is fairly flexible, although this version is the most neutral and natural.

The sentence:

• Этот стул неудобный, поэтому я сижу на диване.

sounds like a standard statement.

You could move things around for emphasis, for example:

• Я поэтому сижу на диване.
• На диване я сижу, потому что этот стул неудобный.

But these alternatives change the emphasis or sound less neutral in everyday speech.

So the original word order is a good default pattern for learners.

Not always. Неудобный can mean:

• physically uncomfortable
• inconvenient
• awkward
• unsuitable in some way

In this sentence, because it describes стул, it most naturally means physically uncomfortable.

But in other contexts:

• неудобное время = an inconvenient time
• неудобный вопрос = an awkward question
• неудобное место = an inconvenient place

So the exact shade of meaning depends on the noun and context.