Вечером мы повесим гирлянду у окна, и в комнате сразу станет уютно и красиво.

Yes. Вечером means in the evening.

Russian often uses the instrumental case for parts of the day with no preposition:

• утром = in the morning
• днём = in the daytime / during the day
• вечером = in the evening
• ночью = at night

So Вечером мы повесим... is a very natural way to say In the evening we’ll hang...

Повесим is the 1st person plural future form of повесить, a perfective verb.

That means:

• мы = we
• повесим = we will hang / we will put up

The perfective aspect is used because the speaker is talking about a completed action in the future: we will put the garland up, and the action will be done.

Compare:

• вешать = to hang, to be hanging, to hang repeatedly/process
• повесить = to hang up successfully, to put up

So повесим гирлянду suggests a one-time completed action: we’ll put up the garland.

Because it is the direct object of the verb повесим.

The dictionary form is:

• гирлянда = garland / string of lights

But after a transitive verb like повесить, a feminine noun in -а usually goes into the accusative singular:

• гирлянда → гирлянду

So:

• повесим гирлянду = we’ll hang the garland / string lights

In modern Russian, гирлянда can mean:

• a decorative garland
• a string of lights
• festive hanging decoration

In this sentence, many learners would understand it as string lights or a decorative garland, especially because it is being hung by the window and it makes the room feel cozy.

У окна means by the window or near the window.

The preposition у normally takes the genitive case, so:

• окно = window
• у окна = by the window / near the window

This does not usually mean on the window. It means the garland is placed at the window area, for example beside it, around it, or near it.

Because в here means in, referring to location, and with location Russian uses the prepositional case.

• комната = room
• в комнате = in the room

So:

• в комнате сразу станет... = in the room it will immediately become...

If в meant motion into the room, you would expect the accusative instead.

Because the sentence has two clauses, each with its own verb:

1. Вечером мы повесим гирлянду у окна
2. и в комнате сразу станет уютно и красиво

The verbs are:

• повесим
• станет

When и joins two full clauses, Russian normally uses a comma before it.

Because станет means will become, while будет means will be.

Here the idea is a change of state:

• first, we hang the garland
• then, the room becomes cozy and beautiful

So станет is the natural choice.

Compare:

• В комнате будет уютно. = It will be cozy in the room.
• В комнате станет уютно. = It will become cozy in the room.

The second one highlights the transformation.

This is a very common impersonal construction in Russian.

English often uses a dummy subject:

• it will become cozy and beautiful in the room

Russian usually does not need that kind of dummy it. The sentence simply states the resulting atmosphere or condition:

• в комнате станет уютно и красиво
  = the room will feel cozy and beautiful / it will become cozy and beautiful in the room

So nothing is missing. Russian grammar allows this naturally.

Because here they are not agreeing with a noun like комната. They are used predicatively, describing the overall state/atmosphere.

So:

• уютная комната = a cozy room
• красивая комната = a beautiful room

But:

• в комнате уютно = it is cozy in the room
• в комнате красиво = it is beautiful in the room

In the sentence, уютно and красиво describe how the room feels, not what noun they modify directly.

Сразу means right away, immediately, or at once.

It suggests that as soon as the garland is hung, the atmosphere changes:

• и в комнате сразу станет уютно и красиво
• and the room will immediately feel cozy and beautiful

Yes, Russian word order is relatively flexible, but this version sounds natural and neutral.

The sentence begins with Вечером to set the time first:

• Вечером = in the evening

Then it gives the subject and action:

• мы повесим гирлянду

Then the location:

• у окна

Then the result:

• и в комнате сразу станет уютно и красиво

This order flows well because it moves from:

1. when
2. who
3. what action
4. where
5. what result

Other word orders are possible, but they may change emphasis.

In this sentence, the most natural reading is that у окна tells you where the garland will be hung.

So the meaning is:

• we’ll hang the garland by the window

Grammatically, Russian can sometimes leave such attachments a little flexible, but context strongly points to the garland’s location, not the people’s location.