Когда занавес снова открылся, декорации уже были совсем другими.

Because Когда занавес снова открылся is a subordinate clause introduced by когда = when.

Russian, like English, normally separates this kind of when-clause from the main clause with a comma:

• Когда занавес снова открылся, ...
• When the curtain opened again, ...

So the comma is required here.

Because занавес is the subject of the verb открылся.

In this sentence, the curtain is the thing that opened, so Russian uses the nominative:

• занавес = the curtain

If it were a direct object, you would expect a different case, but here it is the grammatical subject.

Открылся is the reflexive form of the verb, with -ся added.

Here it means something like:

• opened
• came open

So занавес открылся means the curtain opened.

By contrast:

• открыл = opened something
• for example, Он открыл дверь = He opened the door

So:

• занавес открылся = the curtain opened
• кто-то открыл занавес = someone opened the curtain

In Russian, the reflexive form is very often used when something opens by itself or is presented that way.

Because it agrees with занавес, which is a masculine singular noun.

In the past tense, Russian verbs show gender in the singular:

• masculine: открылся
• feminine: открылась
• neuter: открылось
• plural: открылись

Since занавес is masculine singular, the verb must be открылся.

Снова means again.

So:

• занавес снова открылся = the curtain opened again

Its position is fairly natural here: after the subject and before the verb. Russian word order is flexible, but this placement sounds normal and neutral.

You could move it for emphasis, but занавес снова открылся is the standard wording.

Yes, опять could be used here too.

Both can mean again, but there is often a small nuance:

• снова = again, anew, once more
• опять = again, often with a sense like yet again

In many contexts they are interchangeable. Here:

• Когда занавес снова открылся...
• Когда занавес опять открылся...

Both are possible. Снова can sound a little more neutral or literary, while опять can sometimes sound more conversational.

Уже means already.

It shows that by the time the curtain opened again, the scenery had already changed.

So the structure is:

• Когда занавес снова открылся = when the curtain opened again
• декорации уже были совсем другими = the decorations/scenery were already quite different

It helps show the sequence of events clearly.

Because декорации usually refers to stage decorations, sets, or scenery, and this idea is commonly expressed in the plural in Russian.

So:

• декорация = one decoration / one set element
• декорации = the stage set / scenery / decorations

In a theater context, the plural is very natural because a stage set is made up of multiple visual elements.

Because декорации is plural, so the past-tense form of to be must also be plural:

• singular masculine: был
• singular feminine: была
• singular neuter: было
• plural: были

So:

• декорации были = the decorations/scenery were

Because after быть in this kind of sentence, Russian often uses the instrumental case to describe what something was, became, or seemed.

So:

• были другими = were different

Here, другими is the instrumental plural form of другой.

This is a very common pattern in Russian:

• Он был врачом = He was a doctor
• Они были счастливыми = They were happy
• Декорации были другими = The scenery was different

You may also encounter predicate adjectives in other forms, but instrumental after быть is a standard and important pattern to know.

Совсем here means quite, entirely, or completely.

So:

• другими = different
• совсем другими = quite different / completely different

It strengthens the idea. The sentence is not just saying the scenery was somewhat changed, but that it was really different.

Because it agrees with декорации, which is plural.

Russian adjectives must match the noun they describe in number, gender, and case. Here the noun is plural, and the adjective is in the instrumental plural because of the construction with были:

• nominative plural: другие
• instrumental plural: другими

So декорации уже были совсем другими is grammatically consistent.

Yes. Russian word order is flexible, and you can move elements around to change emphasis.

The original sentence is neutral and natural:

• Когда занавес снова открылся, декорации уже были совсем другими.

But you might also see variations like:

• Когда занавес снова открылся, уже декорации были совсем другими.
• Когда снова открылся занавес, декорации уже были совсем другими.

These are possible, but they sound different in emphasis. For a learner, the original version is the best neutral model to follow.

Yes, in this context занавес most naturally means the theater curtain.

That fits with декорации, which refers to stage scenery. So the whole sentence clearly belongs to a theater scene:

• the curtain opens again
• the scenery is already different

Without context, занавес can simply mean curtain, but here the theatrical meaning is the obvious one.