Learn Hardcore Turkish: Bu koltuk rahat; akşam burada dinleniyorum. - This armchair is comfortable; I rest here in the evening.

In Turkish, a present-tense “to be” verb is usually not written in simple statements.
So Bu koltuk rahat literally works as “This armchair (is) comfortable.”

You can add a copular ending for emphasis or formality:

Bu koltuk rahattır. = “This armchair is (indeed) comfortable.” (more formal/definitive)
Bu koltuk rahat. = neutral, everyday speech

What does koltuk specifically mean here—chair, sofa, or armchair?

Koltuk commonly means an armchair or a comfortable seat (often upholstered). Depending on context it can also be used for a seat (e.g., on a bus/plane), but in a home setting with rahat + dinleniyorum, it strongly suggests armchair rather than a dining chair.

What’s the role of the semicolon (;) in Bu koltuk rahat; akşam burada dinleniyorum?

The semicolon links two closely related independent clauses: 1) Bu koltuk rahat
2) akşam burada dinleniyorum

It’s like saying: “This armchair is comfortable; in the evening I rest here.”
In everyday Turkish you could also use:

a period: Bu koltuk rahat. Akşam burada dinleniyorum.
a comma (more casual): Bu koltuk rahat, akşam burada dinleniyorum.
çünkü (“because”) if you want a clear cause: Bu koltuk rahat, çünkü akşam burada dinleniyorum.

Why is akşam not marked with a case ending? Shouldn’t it mean “in the evening”?

Time words like akşam, sabah, yarın, dün often appear without case endings and still function like English “in the evening / in the morning.”
So akşam here means “in the evening / at night” by default.

If you want “in the evenings (habitually)”, Turkish often uses:

akşamları burada dinleniyorum. = “I rest here in the evenings.”

If you want “this evening/tonight” more explicitly:

bu akşam burada dinleniyorum. = “I’m resting here tonight.”

What exactly is burada—is it a noun with an ending or just an adverb?

Burada is built from bura (“here/this place”) + the locative -da (“in/at”), and it functions as an adverb meaning “here.”
Related forms you’ll see:

burada = “here (in this place)”
buraya = “to here” (direction)
buradan = “from here” (source)

Why is dinleniyorum in the present continuous? Could I use the simple present/aorist instead?

-iyor (present continuous) often implies something like:

what you’re doing now, or
what you do around this period, or
a routine with a vivid “currently/these days” feeling, especially with time words like akşam.

So akşam burada dinleniyorum can mean “I rest here in the evening” (as a regular thing) or “I’m resting here this evening” depending on context.

If you use the aorist (-r/-ar/-er), it sounds more like a general habit/fact:

Akşam burada dinlenirim. = “I rest here in the evenings / I usually rest here.”

How is dinleniyorum built, and what does each part mean?

It breaks down like this:

dinlen- = verb stem meaning “rest” (also “to take a break”)
-iyor = present continuous marker (vowel chosen by harmony)
-um = 1st person singular ending (“I”)

So dinleniyorum = “I am resting / I rest (these days/this evening).”

Why is the vowel in -iyor specifically i here (not -uyor/-ıyor/-üyor)?

This is vowel harmony. The vowel in -(I)yor changes depending on the last vowel of the stem.
The last vowel in dinlen- is e (a front vowel), so the suffix takes a front high vowel: i.

Compare:

geliyorum (gel- has e)
bakıyorum (bak- has a, so -ıyor)
oturuyorum (otur- has u, so -uyor)
görüyorum (gör- has ö, so -üyor)

Why isn’t the subject ben (“I”) written?

Turkish usually drops subject pronouns because the verb ending already shows the person.
dinleniyorum already means “I am resting.” Adding ben is possible but usually adds emphasis/contrast:

Ben akşam burada dinleniyorum. = “I rest here in the evening (as opposed to someone else).”

Is the word order fixed? Could I move akşam or burada around?

Word order is flexible, and moving words changes focus/emphasis more than basic meaning. For example:

Akşam burada dinleniyorum. (neutral)
Burada akşam dinleniyorum. (focus on “here” a bit more)
Akşam dinleniyorum burada. (more conversational; “here” feels tacked on)
Burada dinleniyorum akşam. (emphasis on “here,” then adds “in the evening”)

The most neutral, textbook-like order is often: time + place + verb.

Does rahat behave like an adjective or can it also act like a noun?

In Bu koltuk rahat, rahat is an adjective meaning “comfortable.”
But Turkish also uses rahat in other ways:

as an adverb: Rahat konuş. = “Speak comfortably / Relax and speak.”
in set phrases: rahat et- = “to relax,” rahat bırak- = “leave (someone) alone”
as a noun-like idea in context: rahat = “comfort/ease” (depending on sentence)

Here, it’s plainly adjectival.

Could I also say Bu koltuk rahattır or Bu koltuk rahat oluyor? What’s the difference?

Bu koltuk rahat. = simple, natural statement.
Bu koltuk rahattır. = more formal or emphatic (“It is definitely comfortable.”)
Bu koltuk rahat oluyor. = sounds like “It’s becoming comfortable / It turns out comfortable,” often used when comfort is conditional or discovered over time (less natural if you just mean a stable property).