Хотя вечер был долгим, я не могу не думать о том, как красиво светился занавес в конце.

Хотя means although / even though. It introduces a concessive clause: a clause that presents a contrast.

So:

• Хотя вечер был долгим = Although the evening was long
• main clause: я не могу не думать... = I can’t help thinking...

Russian often puts a comma between the хотя clause and the main clause.

Because Хотя вечер был долгим is a subordinate clause, and Russian separates it from the main clause with a comma.

Structure:

• Хотя вечер был долгим, = subordinate clause
• я не могу не думать... = main clause

This is very standard Russian punctuation.

Here долгим is in the instrumental case. After past-tense быть (был, была, было, были), Russian very often uses the instrumental for a predicate noun or adjective.

So:

• вечер был долгим = the evening was long

You may also see nominative in some contexts, but instrumental is very common and neutral here.

A useful pattern:

• Он был усталым
• День был трудным
• Вечер был долгим

For a learner, it is safest to treat past/future быть + predicate adjective/noun as a place where instrumental is often expected.

Yes, it is a double negative on the surface, but this is a normal Russian pattern.

не могу не + infinitive means:

• I can’t help but...
• I can’t not...
• I cannot help...

So:

• я не могу не думать = I can’t help thinking

It does not mean a logical positive in the way an English learner might expect. It is an idiomatic construction.

More examples:

• Я не могу не улыбаться = I can’t help smiling
• Он не мог не заметить = He couldn’t fail to notice

Because думать describes an ongoing mental process: thinking, dwelling on something, having it on your mind.

In не могу не думать, the idea is not a single completed thought, but continued mental activity.

So imperfective думать is the natural choice.

Because думать о means to think about something.

Here, о том works like about that / about the fact / about how and introduces the content of the thought:

• думать о том, как красиво светился занавес
  = to think about how beautifully the curtain glowed

If you said думать как... without о том, it would usually sound incomplete or suggest a different meaning, often closer to thinking how to do something.

So о том, как... is a very common Russian structure for introducing a subordinate clause after a preposition.

Том is the prepositional singular form of то.

That happens because of the preposition о, which usually requires the prepositional case:

• о том = about that

So the structure is:

• думать о том = to think about that
• думать о том, как... = to think about how...

This is a common pattern:

• говорить о том, что... = to talk about the fact that...
• думать о том, почему... = to think about why...
• вспоминать о том, как... = to remember how...

Because как красиво светился занавес is a subordinate clause connected to о том.

So the sentence breaks like this:

• я не могу не думать о том,
• как красиво светился занавес в конце

Russian normally uses a comma before this kind of clause.

Here как means something like how in how beautifully the curtain glowed.

Because красиво is an adverb, and it modifies the verb светился.

It describes how the curtain glowed:

• красиво светился = glowed beautifully

If you wanted to describe the curtain itself as beautiful, you would use an adjective:

• красивый занавес = a beautiful curtain

But here the focus is on the manner of glowing, so the adverb красиво is correct.

Светился means was glowing / glowed / gave off light.

The verb светиться often suggests that something itself is shining or glowing.

Compare roughly:

• светить = to shine, to cast light, sometimes to light something
• светиться = to glow, to be lit up, to shine visibly

So занавес светился gives the image that the curtain itself appeared luminous or illuminated.

The -ся here is just part of the normal verb form светиться. It does not mean reflexive in the English sense of it shines itself.

In Russian past tense, the verb agrees in gender and number with the subject.

• вечер is masculine singular, so: был
• занавес is masculine singular, so: светился

Compare:

• ночь была долгой — ночь is feminine
• окно светилось — окно is neuter
• занавесы светились — plural

So the endings help show the gender/number of the subject.

Russian word order is flexible, and this order helps emphasize the visual impression first:

• как красиво светился... = the sentence first highlights how beautifully it glowed
• then comes занавес = the curtain

This sounds natural and slightly expressive. It draws attention to the glowing before naming the object.

A more rigidly English-like order is possible in some contexts, but Russian often moves words around for emphasis or style.

Most naturally, в конце means at the end.

Depending on the wider context, that could mean:

• at the end of the evening
• at the end of the performance
• in the final part of the event

So in this sentence it probably means that the curtain glowed beautifully at the end of whatever event is being described.

If the speaker wanted to make it more explicit, they could say:

• в конце вечера = at the end of the evening
• в конце спектакля = at the end of the performance

By itself, в конце is understandable, but a little context-dependent.

No. Here как is not asking a question.

It means how in the sense of how beautifully:

• как красиво светился занавес = how beautifully the curtain glowed

So this is an embedded exclamatory idea, not a direct question.

Compare:

• Как красиво! = How beautiful!
• Я помню, как красиво это было. = I remember how beautiful it was.

In your sentence, как красиво expresses admiration, not a request for information.