Мне удобно сидеть в этом кресле.

Because this sentence uses an impersonal construction.

Russian often says things like:

• Мне удобно = It is comfortable/convenient for me
• Мне трудно = It is difficult for me
• Мне холодно = I am cold / It feels cold to me

So мне is in the dative case, showing the person who experiences the feeling or situation.
Literally, the structure is closer to To me, it is comfortable to sit in this armchair than to I sit comfortably.

Удобно means comfortable, convenient, or easy to use/be in, depending on context.

In this sentence, it describes the situation as comfortable for the speaker. Grammatically, words like удобно, трудно, легко, холодно, жарко are often treated as predicative words or category-of-state words in Russian. They commonly appear in impersonal sentences.

So here:

• мне = for me
• удобно = comfortable/convenient
• сидеть = to sit

Together: It is comfortable for me to sit...

In the present tense, Russian usually omits the verb to be.

So where English says:

• It is comfortable
• This is a chair

Russian often just says:

• удобно
• это кресло

That is why Мне удобно сидеть... has no separate word for is.

If you put the sentence in the past or future, you would see a form of быть:

• Мне было удобно сидеть в этом кресле. = It was comfortable for me to sit in this armchair.
• Мне будет удобно сидеть в этом кресле. = It will be comfortable for me to sit in this armchair.

Because the sentence is talking about the activity itself: to sit.

After words like удобно, трудно, легко, Russian very often uses an infinitive to say what action is easy, difficult, comfortable, etc.

Compare:

• Мне удобно сидеть. = It is comfortable for me to sit.
• Мне трудно читать. = It is difficult for me to read.
• Ему легко понять. = It is easy for him to understand.

So сидеть is not a tense form here; it just names the action.

It is the prepositional case after в because it expresses location: sitting in this armchair.

Base form:

• это кресло = this armchair

After в for location:

• в этом кресле = in this armchair

Both words change as needed:

• этом matches кресле
• кресло is neuter singular, so the demonstrative is этом

So:

• nominative: это кресло
• prepositional: в этом кресле

Because кресло is a neuter noun.

Its basic form is:

• кресло = armchair

The demonstrative this must agree with it in gender, number, and case.

So:

• masculine: этот
• feminine: эта
• neuter: это

But after в with location, both words go into the prepositional case:

• masculine/neuter prepositional singular: этом
• feminine prepositional singular: этой

Since кресло is neuter, we get:

• в этом кресле

Because Russian normally says:

• сидеть в кресле = sit in an armchair
• сидеть на стуле = sit on a chair

An armchair is conceptualized more like something you sit in, while a regular chair or stool is something you sit on.

So these are the usual pairings:

• на стуле = on a chair
• на диване = on a sofa
• в кресле = in an armchair

This is one of those collocation habits that learners simply need to get used to.

Yes, absolutely. That is very natural.

• Мне удобно в этом кресле. = I’m comfortable in this armchair.
• Мне удобно сидеть в этом кресле. = It’s comfortable for me to sit in this armchair.

The version with сидеть is more explicit: it specifically says that the act of sitting there is comfortable.

Without сидеть, the sentence is a bit broader and can just describe the overall feeling of being in the chair.

The two sentences are related, but they are not the same.

Мне удобно сидеть в этом кресле

• focuses on the speaker’s experience
• means the chair is comfortable/suitable for sitting
• often sounds more general

Я удобно сижу в этом кресле

• means I am sitting comfortably in this armchair
• describes the speaker’s current posture or manner of sitting
• sounds more like an ongoing physical situation right now

So if you are talking about whether the chair suits you, Мне удобно сидеть... is usually the better choice.

It is impersonal.

There is no normal grammatical subject like я. Instead, Russian presents the situation as a state:

• мне удобно = it is comfortable for me

This kind of structure is extremely common in Russian. English often uses personal subjects where Russian prefers impersonal wording.

Other examples:

• Мне интересно. = I’m interested.
• Ему трудно работать. = It is hard for him to work.
• Нам хорошо здесь. = We feel good here.

So this sentence is a very typical Russian pattern.

Yes. Russian word order is flexible, and changing it changes the emphasis, not the basic meaning.

Neutral order:

• Мне удобно сидеть в этом кресле.

Emphasis on the place:

• В этом кресле мне удобно сидеть.

Possible but more marked:

• Сидеть в этом кресле мне удобно.

The first version is the most neutral and natural if you are just stating the fact.

Both can relate to comfort, but they are not always identical.

Удобно often means:

• physically comfortable
• convenient
• practical
• suitable for an action

Комфортно often suggests:

• overall comfort
• a pleasant, comfortable feeling
• sometimes a slightly more modern or stylistic tone

In this sentence, удобно is very natural because the sentence is about how easy/comfortable it is to sit in the chair.

• Мне удобно сидеть в этом кресле. = very standard
• Мне комфортно в этом кресле. = also possible, but slightly different in feel

You simply change удобно to неудобно:

• Мне неудобно сидеть в этом кресле.

That means:

• it is uncomfortable for me to sit in this armchair
• this armchair is not comfortable for sitting

This is a very common pattern:

• Мне удобно → Мне неудобно
• Мне легко → Мне нелегко
• Мне приятно → Мне неприятно

Usually, yes.

Кресло normally means an armchair or a large comfortable chair, often with armrests.
A regular chair is usually стул.

So:

• стул = chair
• кресло = armchair / easy chair

That matters here because сидеть в кресле sounds natural specifically with кресло.