Bu sandalye rahatsız, bu yüzden ben koltukta oturuyorum.

In Turkish, the verb to be is often omitted in the simple present when making a basic statement.

So:

• Bu sandalye rahatsız = This chair is uncomfortable

There is no separate present-tense word like English is here. Turkish simply puts the subject and the description together.

You will see this a lot:

• Hava güzel = The weather is nice
• O yorgun = He/She is tired
• Ben hazırım = I am ready

In other tenses, Turkish does show this more clearly:

• Bu sandalye rahatsızdı = This chair was uncomfortable

Bu yüzden means therefore, so, because of this, or for this reason.

It links the first idea to the result:

• Bu sandalye rahatsız = this chair is uncomfortable
• bu yüzden = for that reason / so
• ben koltukta oturuyorum = I am sitting on the armchair/sofa

So bu yüzden introduces the consequence of what was just said.

Other common Turkish connectors with a similar job are:

• bu nedenle = for this reason, therefore
• o yüzden = that’s why, so

That is true: Turkish often leaves out subject pronouns because the verb ending already shows the person.

So:

• Koltukta oturuyorum already means I am sitting on the armchair/sofa

The ending -yorum tells you the subject is I.

Here, ben is included for emphasis or contrast. It can suggest something like:

• I’m sitting on the armchair
• As for me, I’m sitting on the armchair
• I am sitting on the armchair instead

In this sentence, ben may sound slightly contrastive, as if the speaker is making a choice because the chair is uncomfortable.

The ending -ta is the locative case, which usually means in, on, or at.

So:

• koltuk = armchair / couch / seat
• koltukta = on the armchair / in the armchair / on the seat

In this sentence:

• koltukta oturuyorum = I am sitting on the armchair

Why -ta and not -da?

Turkish has consonant harmony here:

• after a voiceless consonant like k, the locative becomes -ta / -te
• after a voiced consonant or vowel, it becomes -da / -de

Examples:

• evde = at home
• okulda = at school
• parkta = in the park
• koltukta = on the armchair

Oturuyorum is the present continuous form of oturmak (to sit / to be sitting).

It breaks down like this:

• otur- = verb stem
• -uyor- = present continuous marker
• -um = I

So:

• oturuyorum = I am sitting

Turkish often uses the present continuous where English might use either:

• I am sitting
• I sit
• I’m seated

That is why oturuyorum sounds natural here.

By contrast:

• otururum usually means I sit or I usually sit
• it can sound habitual rather than describing what is happening now

So in this sentence, oturuyorum is the natural choice for a current action or situation.

The dictionary form is oturmak.

To make the present continuous:

1. Remove -mak
   • oturmak → otur-
2. Add the present continuous suffix
   • -iyor / -ıyor / -uyor / -üyor depending on vowel harmony
   • here it becomes -uyor
3. Add the personal ending for I
   • -um

So:

• otur- + uyor + um → oturuyorum

A few more examples:

• gelmek → geliyorum = I am coming
• bakmak → bakıyorum = I am looking
• yürümek → yürüyorum = I am walking

This is a very common vocabulary question.

• sandalye = chair
• koltuk = armchair, easy chair, sofa seat, or sometimes just a more comfortable cushioned seat

In this sentence, the contrast matters:

• Bu sandalye rahatsız = This chair is uncomfortable
• bu yüzden ben koltukta oturuyorum = so I’m sitting in/on the armchair instead

So koltuk suggests a more comfortable seat than sandalye.

Depending on context, koltuk can also mean:

• a seat in a vehicle or theater
• a couch/sofa place
• even a figurative position or post

But here it clearly means a comfortable seat.

Here, rahatsız means uncomfortable.

So:

• Bu sandalye rahatsız = This chair is uncomfortable

But rahatsız is a flexible word in Turkish. Depending on context, it can also mean:

• uncomfortable
• disturbed
• bothered
• unwell / ill

Examples:

• Rahatsızım = I’m uncomfortable / I’m unwell
• Beni rahatsız ediyor = It is bothering me

In your sentence, because it describes a chair, the natural meaning is uncomfortable.

Yes, Turkish word order is flexible, but this sentence uses a very natural order.

The second clause is:

• ben koltukta oturuyorum

Literally:

• I on-the-armchair am sitting

A common neutral Turkish pattern is:

• subject + place + verb

So this is perfectly normal.

You could also say:

• Koltukta oturuyorum
• Ben oturuyorum koltukta
• Bu yüzden koltukta oturuyorum

But the emphasis changes.

For example:

• Koltukta oturuyorum puts less attention on I
• Ben koltukta oturuyorum emphasizes the subject a bit more
• Turkish often places the most important or newest information just before the verb

So the original sentence sounds natural and clear.

Because it refers to two different things in the sentence structure.

First:

• Bu sandalye = this chair

Then:

• bu yüzden = because of this / for this reason

In the second case, bu does not describe a noun like chair. Instead, it refers back to the whole previous idea: the fact that this chair is uncomfortable.

So the two bu words do different jobs:

• Bu sandalye = demonstrative adjective, modifying sandalye
• bu yüzden = part of a fixed expression meaning for this reason

Bu yüzden is neutral and very common. It works in both speech and writing.

Some similar expressions are:

• o yüzden = so / that’s why
  Very common in everyday speech.
• bu nedenle = therefore / for this reason
  Slightly more formal.
• ondan dolayı = because of that
  Common in speech.

So in everyday Turkish:

• Bu sandalye rahatsız, o yüzden koltukta oturuyorum
  is also very natural.

Your sentence with bu yüzden is completely correct and natural.

Yes, in casual writing you may sometimes see it without a comma, but the comma is helpful because it separates the two clauses:

• Bu sandalye rahatsız, bu yüzden ben koltukta oturuyorum.

The comma makes the logic clearer:

1. the chair is uncomfortable
2. therefore I’m sitting on the armchair

In careful writing, using the comma here is a good choice.

Not if you want the same meaning.

• Bu sandalye rahatsız = This chair is uncomfortable
• Bu sandalyede... = On this chair / in this chair...

So sandalyede would need a different structure, because it means in/on this chair.

For example:

• Bu sandalyede rahat oturamıyorum = I can’t sit comfortably on this chair

But in your original sentence, sandalye is the subject, so it stays in its basic form with no case ending.

Usually oturmak means to sit, to be sitting, or sometimes to live/reside depending on context.

In this sentence:

• koltukta oturuyorum = I am sitting on/in the armchair

If you want the idea of sit down as a movement, Turkish often still uses oturmak, but context tells you whether it means the action of sitting down or the state of being seated.

Examples:

• Lütfen oturun. = Please sit down / Please have a seat.
• Koltukta oturuyorum. = I am sitting on the armchair.

So oturmak can cover both ideas, but here it clearly means to be sitting.