Сегодня в комнате довольно тихо.

In Russian, the verb быть (to be) is normally omitted in the present tense.

So:

• Сегодня в комнате довольно тихо. literally looks like
• Today in the room fairly quiet.

But it naturally means:

• Today it is fairly quiet in the room.

In the past or future, Russian does use forms of быть:

• В комнате было тихо. — It was quiet in the room.
• В комнате будет тихо. — It will be quiet in the room.

Because after в when it means in and shows location, Russian uses the prepositional case.

• комната = room (dictionary form, nominative)
• в комнате = in the room (prepositional singular)

So:

• в комнате = in the room

A useful comparison:

• в комнате — in the room (location)
• в комнату — into the room (motion toward, accusative)

Here тихо is not an adjective agreeing with комната. It is being used as a predicative adverb/state word, meaning something like quiet or it is quiet.

So the sentence is not saying:

• the room is a quiet one in an adjective-agreement way

It is saying:

• it is quiet in the room

Compare:

• тихая комната — a quiet room
  Here тихая is an adjective describing the noun комната.

• В комнате тихо. — It is quiet in the room.
  Here тихо describes the situation/state.

This is a very common Russian pattern:

• Здесь холодно. — It is cold here.
• Дома тепло. — It is warm at home.
• На улице шумно. — It is noisy outside.

Довольно here means fairly, quite, or rather.

So:

• довольно тихо = fairly quiet / quite quiet / rather quiet

It softens the statement compared with очень (very).

Compare:

• тихо — quiet
• довольно тихо — fairly quiet
• очень тихо — very quiet

Be aware that довольно can have other meanings in other contexts, including enough, but in this sentence it means fairly/quite.

Сегодня means today and functions as an adverb of time.

It tells you when the situation is true:

• Сегодня в комнате довольно тихо. — Today it is fairly quiet in the room.

Russian often places time words near the beginning of the sentence, but the position can change depending on emphasis.

No, Russian word order is fairly flexible. The basic meaning stays the same, but the emphasis can shift.

All of these are possible:

• Сегодня в комнате довольно тихо.
• В комнате сегодня довольно тихо.
• Довольно тихо сегодня в комнате.

The version you have is natural because it starts with the time frame сегодня and then gives the location в комнате.

A rough feel for the emphasis:

• Сегодня в комнате довольно тихо. — neutral; today is the starting point
• В комнате сегодня довольно тихо. — focuses first on in the room
• Сегодня довольно тихо в комнате. — a bit more emphasis on fairly quiet

Yes, this is essentially an impersonal construction.

There is no explicit subject like it in English. Russian often does this with words like:

• тихо — quiet
• холодно — cold
• жарко — hot
• темно — dark

English needs a dummy subject:

• It is quiet in the room.

Russian does not:

• В комнате тихо.

So there is no real subject word here; the sentence simply states a condition or atmosphere.

Not really. Even though тихо can sometimes correspond to English quietly, that would be misleading here.

In this sentence, тихо means quiet as a state, not quietly as a manner of action.

So:

• В комнате тихо. = It is quiet in the room.

But:

• Он тихо говорит. = He speaks quietly.

That is an important distinction:

• with a verb of action, тихо can mean quietly
• in a sentence like this, it means it is quiet

Russian often distinguishes location and motion through case.

With в:

• в + prepositional = in a place, already there
• в + accusative = into a place, moving there

So:

• в комнате — in the room
• в комнату — into the room

Since the sentence describes a state (it is fairly quiet), not movement, в комнате must be location.

The main word stress is:

• сего́дня
• в ко́мнате
• дово́льно
• ти́хо

So a stress guide would look like:

• сего́дня в ко́мнате дово́льно ти́хо

A rough English-style approximation is:

• syi-GOD-nya v KOM-na-tye da-VOL-na TEE-kha

But it is best to learn the real Russian vowel reduction from audio, since unstressed vowels are not pronounced exactly the way they are written.

Yes. That would be perfectly natural.

• довольно тихо = fairly / quite quiet
• очень тихо = very quiet

So the difference is just strength:

• Сегодня в комнате довольно тихо. — Today it’s fairly quiet in the room.
• Сегодня в комнате очень тихо. — Today it’s very quiet in the room.

Довольно sounds milder and more moderate than очень.