Learn Hardcore German: Dieser Stuhl ist unbequem, aber ich bleibe trotzdem sitzen. - This chair is uncomfortable, but I stay seated anyway.

der Stuhl = the chair (neutral, just identifying a specific chair already known from context).
dieser Stuhl = this chair (more pointing, drawing attention to this particular one, often with a contrast: this one, not another).

In the sentence „Dieser Stuhl ist unbequem …“, the speaker is clearly picking out one specific chair, probably contrasting it with other chairs that might be comfortable.

Why does unbequem have no ending here? Why not unbequemer or unbequeme?

Because unbequem is used after the verb sein (“to be”), as a predicative adjective:

Dieser Stuhl ist unbequem. – This chair is uncomfortable.

In German, adjectives used after verbs like sein, werden, bleiben do not take endings:

Das Essen ist gut. (not gute)
Er bleibt ruhig. (not ruhiger here)

Adjective endings appear when the adjective is before a noun:

ein unbequemer Stuhl – an uncomfortable chair
der unbequeme Stuhl – the uncomfortable chair

So:

vor dem Nomen: unbequemer / unbequeme / unbequemes … (with endings)
nach „sein/werden/bleiben“: unbequem (no ending)

What does trotzdem mean exactly, and where does it go in the sentence?

trotzdem is a sentence adverb meaning “nevertheless, anyway, despite that”.

Typical positions:

In the middle field (neutral):
- Ich bleibe trotzdem sitzen.
  (subject – verb – trotzdem – rest)
At the beginning (emphatic):
- Trotzdem bleibe ich sitzen.
  Here, trotzdem is in position 1, so the finite verb (bleibe) must be in position 2.
It cannot go at the very end like in English “I stay seated anyway”:
- ❌ Ich bleibe sitzen trotzdem. (wrong)

So you either put trotzdem in the middle or at the front, but not at the end.

Why does the sentence use both aber and trotzdem? Isn’t that redundant?

It’s a bit emphatic, but it’s normal and idiomatic.

aber = “but / however” → marks the contrast between the two clauses:
- Dieser Stuhl ist unbequem, aber … – This chair is uncomfortable, but …
trotzdem = “nevertheless / anyway / in spite of that” → adds extra emphasis that the second action happens despite the first statement:
- … ich bleibe trotzdem sitzen. – … I stay seated anyway.

You could say:

Dieser Stuhl ist unbequem, aber ich bleibe sitzen.
Dieser Stuhl ist unbequem. Trotzdem bleibe ich sitzen.

Using both gives a strong contrast:

The chair is uncomfortable, but I still stay seated anyway.

Could I express the same idea with obwohl? How would the sentence change?

Yes. With obwohl (“although/though”), you make a subordinate clause:

Obwohl der Stuhl unbequem ist, bleibe ich sitzen.
(Although the chair is uncomfortable, I stay seated.)

or:

Ich bleibe sitzen, obwohl der Stuhl unbequem ist.

Key differences:

aber + trotzdem → two main clauses joined by aber, with an adverb:
- Dieser Stuhl ist unbequem, aber ich bleibe trotzdem sitzen.
obwohl → one subordinate clause
- one main clause:
  - Obwohl der Stuhl unbequem ist, bleibe ich sitzen.

The meaning is basically the same; obwohl sounds a bit more formal/structured, while aber / trotzdem feels more conversational.

Why do we say ich bleibe sitzen and not just ich sitze?

ich sitze = “I am sitting” (just describing your current position).
ich bleibe sitzen = “I stay seated / I remain sitting / I don’t get up”.

So bleiben sitzen adds the idea of continuation and intent:
you are already sitting and you decide not to stand up, even though the chair is uncomfortable.

In the context of the sentence:

Dieser Stuhl ist unbequem, aber ich sitze.
→ grammatically OK, but it just says I am sitting.
Dieser Stuhl ist unbequem, aber ich bleibe sitzen.
→ emphasizes: I keep sitting, I won’t get up.

What’s the difference between sitzen, setzen, and sich setzen?

They’re easy to mix up:

sitzen – to be in a sitting position (no movement)
- Ich sitze auf dem Stuhl. – I am sitting on the chair.
setzen – to set/put something (or someone) down (transitive, needs an object)
- Ich setze das Kind auf den Stuhl. – I put the child on the chair.
sich setzen – to sit down (movement, reflexive)
- Ich setze mich auf den Stuhl. – I sit down on the chair.

In ich bleibe sitzen, we’re talking about continuing to be in a seated position (so sitzen, not sich setzen).

Is bleiben here like an auxiliary verb (like English “keep sitting”), or a full verb?

Here bleiben is a full verb meaning “to stay / to remain”.
The combination bleiben + sitzen is a verb phrase meaning “to remain seated”.

ich bleibe – I stay
ich bleibe sitzen – I stay seated

It’s not a grammatical auxiliary like haben or sein used for forming tenses.
In the past tense:

Ich bin sitzen geblieben. – I stayed seated.

You can even find it written together as sitzenbleiben, especially as a dictionary entry, but in a sentence you normally write it as bleibe sitzen.

Can I say Trotzdem bleibe ich sitzen instead of ich bleibe trotzdem sitzen? Is there a difference?

Yes, both are correct:

Ich bleibe trotzdem sitzen.
→ neutral word order, very common.
Trotzdem bleibe ich sitzen.
→ puts more emphasis on trotzdem (“still / in spite of that”).

The meaning doesn’t really change; the second version just highlights the “nevertheless” more strongly.

Why Dieser Stuhl ist unbequem and not Dieser Stuhl ist nicht bequem? Are both correct?

Both are grammatically correct, but there’s a nuance:

unbequem = “uncomfortable” (a positive negative quality)
nicht bequem = “not comfortable” (weaker, could sound more neutral)

So:

Dieser Stuhl ist unbequem.
→ clear, direct: the chair is uncomfortable.
Dieser Stuhl ist nicht bequem.
→ softer: the chair is not really comfortable; it may be slightly uncomfortable or just not as comfy as expected.

In everyday speech, unbequem is more natural if you find it clearly uncomfortable.

What does the form dieser tell me about gender and case?

dieser is the masculine nominative singular form of dieser (“this”).

The pattern for dieser in the singular is:

masculine:
- Nominative: dieser Stuhl – this chair
- Accusative: diesen Stuhl
- Dative: diesem Stuhl
feminine:
- Nominative: diese Lampe – this lamp
- Accusative: diese Lampe
- Dative: dieser Lampe
neuter:
- Nominative: dieses Buch – this book
- Accusative: dieses Buch
- Dative: diesem Buch

In Dieser Stuhl ist unbequem, Stuhl is:

masculine (der Stuhl),
in the nominative (subject of the sentence),
so we use dieser.