Jest za późno, żeby zaczynać nowy temat dziś wieczorem.

Because późno here is not an adjective agreeing with a noun. It is an adverb/predicative form used in impersonal statements like:

• Jest późno — It’s late.
• Jest wcześnie — It’s early.
• Jest zimno — It’s cold.

In other words, Polish treats this as a general situation, not as a noun being described.

If you used an adjective, you would need a noun, for example:

• To jest późna pora. — It is a late hour.

So in your sentence, jest za późno is the normal structure.

Because Polish does not need a dummy subject like English it.

English says It is late, but Polish can simply say:

• Jest późno.
• Jest za późno.

That is a complete sentence.

You can say To jest za późno, but that usually sounds more like That is too late or adds contrast/emphasis. In your sentence, plain Jest za późno is the most natural neutral version.

Here za means too, in the sense of more than is acceptable/desirable.

This is a very common pattern:

• za drogo — too expensive
• za szybko — too fast
• za wcześnie — too early
• za późno — too late

So za późno is the normal Polish way to say too late.

A close alternative is zbyt późno, which is a bit more formal or bookish.

Because żeby introduces a subordinate clause, and in standard Polish that clause is separated by a comma.

So:

• Jest za późno, żeby zaczynać nowy temat dziś wieczorem.

This is just regular Polish punctuation. English often does not use a comma before to start, but Polish does before żeby.

In this sentence, żeby introduces the idea corresponding to English to in the pattern too ... to ....

So:

• za późno, żeby zaczynać...

means something like:

• too late to start...

Very literally, it is closer to too late for starting / in order to start, but in natural English the best match is simply too late to start.

A slightly more formal alternative is aby:

• Jest za późno, aby zaczynać nowy temat...

But żeby is more common in everyday Polish.

This is an aspect question.

• zaczynać = imperfective
• zacząć = perfective

In your sentence, zaczynać sounds natural because the speaker is talking about the general idea of getting into starting a new topic at this point in the evening. It has a slightly more process-oriented or less sharply bounded feel.

However, zacząć is also possible:

• Jest za późno, żeby zacząć nowy temat dziś wieczorem.

That version focuses a bit more on the one-time act of beginning.

So the sentence with zaczynać is not the only grammatical choice, but it gives a slightly more natural “it’s too late to be starting...” kind of feeling.

Because zaczynać takes a direct object, and that object goes in the accusative case.

Here, temat is a masculine inanimate noun. For masculine inanimate nouns in the singular, the accusative is the same as the nominative.

So:

• nominative: nowy temat
• accusative: nowy temat

That is why the form does not visibly change.

If it were a different noun type, you would see a clearer change, for example:

• zaczynać nową rozmowę — to start a new conversation

Wieczorem is the instrumental singular form of wieczór, but in this kind of expression it functions adverbially and means in the evening.

So:

• wieczorem — in the evening
• dziś wieczorem — this evening / tonight

This is a very common Polish time expression, similar to:

• rano — in the morning
• nocą — at night / by night
• latem — in summer

So you should learn dziś wieczorem as a standard chunk meaning this evening.

Yes. Dziś and dzisiaj both mean today.

So both are correct:

• dziś wieczorem
• dzisiaj wieczorem

The difference is mostly one of style and rhythm:

• dziś is shorter
• dzisiaj is a bit fuller in sound

In everyday Polish, both are very common.

No, Polish word order is fairly flexible, though the version you have is very natural and neutral:

• Jest za późno, żeby zaczynać nowy temat dziś wieczorem.

You could move things around for emphasis, for example:

• Dziś wieczorem jest za późno, żeby zaczynać nowy temat.
  Emphasizes this evening.

• Jest za późno dziś wieczorem, żeby zaczynać nowy temat.
  Also possible, but less neutral.

Polish often puts the time expression at the end when it is just part of the background information, which is what happens here.

Yes, especially in conversation.

• Za późno, żeby zaczynać nowy temat dziś wieczorem.

This sounds like a shortened, more spoken version of the full sentence. Polish often drops jest in informal speech when the meaning is obvious.

Still, the full version with jest is the safer neutral form, especially for learners and in writing:

• Jest za późno, żeby zaczynać nowy temat dziś wieczorem.

Usually yes in this kind of sentence, but the literal structure is a little different.

• dziś = today
• wieczorem = in the evening

So literally it is today in the evening, but idiomatically English usually says this evening or tonight.

In practice, for your sentence, this evening and tonight are both natural translations depending on context.