A não ser que eu mude o tema hoje, vou terminar o rascunho à noite.

It means unless.

Literally, it is something like if it is not the case that, but in natural English you usually translate it as unless.

So:

A não ser que eu mude o tema hoje
= Unless I change the topic today

This expression is very common in Portuguese and is normally followed by the subjunctive.

Because a não ser que triggers the subjunctive mood.

Here, mude is the present subjunctive form of mudar for eu.

• eu mudo = I change / I do change
• eu mudarei = I will change
• eu mude = present subjunctive, used after expressions like a não ser que

The subjunctive is used because the action is uncertain, hypothetical, or dependent on a condition:

A não ser que eu mude o tema hoje...
= Unless I change the topic today...

It is not saying that the speaker definitely will change it, only that this is a possible condition.

Mudar is a regular -ar verb, so its present subjunctive is formed like this:

• eu mude
• você/ele/ela mude
• nós mudemos
• vocês/eles/elas mudem

A quick way to get the present subjunctive for regular verbs is to start from the eu form of the present indicative:

• present indicative: eu mudo
• remove the final -o
• add subjunctive endings for -ar verbs:
  • -e, -es, -e, -emos, -em

So mudo becomes mude.

Yes, it can be omitted.

Portuguese often drops subject pronouns when the verb ending already makes the subject clear. So these are both correct:

• A não ser que eu mude o tema hoje...
• A não ser que mude o tema hoje...

Including eu can make the sentence a little clearer or slightly more emphatic. In everyday Brazilian Portuguese, speakers often include pronouns more than in some other varieties, but omission is still very common.

It depends on context.

Tema can mean:

• theme
• topic
• subject

In this sentence, topic or subject is often the most natural translation if someone is writing something and may change what they are writing about.

For example:

• in an essay context: topic
• in a broader creative or academic context: theme

So mudar o tema could mean change the topic or change the theme, depending on what is being drafted.

Vou terminar is the very common ir + infinitive future construction in Brazilian Portuguese. It is the most natural choice in everyday speech and very common in writing too.

• vou terminar = I’m going to finish / I will finish
• terminarei = I will finish

Both are correct, but they feel a little different:

• vou terminar sounds more conversational and natural in everyday Brazilian Portuguese
• terminarei sounds a bit more formal, literary, or deliberate

So this sentence sounds very natural as:

vou terminar o rascunho à noite

Rascunho means draft.

It refers to a preliminary version of a text, document, essay, article, or similar piece of writing.

Examples:

• fazer um rascunho = to make a draft
• o primeiro rascunho = the first draft
• terminar o rascunho = to finish the draft

It can also sometimes refer to rough notes or a rough version, depending on context.

À noite means at night or tonight, depending on context.

The à is a contraction of a + a and it takes a grave accent, called crase.

In time expressions, this is very common:

• à noite = at night
• à tarde = in the afternoon
• às 8 horas = at 8 o’clock

In this sentence, vou terminar o rascunho à noite most naturally means I’ll finish the draft tonight or at night.

A useful contrast:

• de noite also exists and can mean at night, often in a more general sense
• à noite is extremely common when talking about when something happens

Yes, hoje can move.

In the sentence:

A não ser que eu mude o tema hoje...

hoje modifies mude and means the change would happen today.

You could also say:

• A não ser que eu hoje mude o tema... — grammatical, but less natural
• A não ser que hoje eu mude o tema... — possible, with more emphasis on today
• A não ser que eu mude hoje o tema... — possible, but less natural in everyday speech

The original word order is very natural in Brazilian Portuguese.

Because the first part is a conditional subordinate clause placed before the main clause.

Structure:

• A não ser que eu mude o tema hoje, = subordinate clause
• vou terminar o rascunho à noite. = main clause

In English, you also commonly use a comma in this kind of structure:

Unless I change the topic today, I’ll finish the draft tonight.

If the order were reversed, the comma would often disappear:

Vou terminar o rascunho à noite, a não ser que eu mude o tema hoje.

That version is also grammatical, though the original order is more straightforward.

Yes. A common alternative is a menos que, which also means unless and also takes the subjunctive.

So you could say:

A menos que eu mude o tema hoje, vou terminar o rascunho à noite.

This means essentially the same thing.

Both are common, though a menos que may be the first form many learners encounter. A não ser que is also very natural and frequent.

Yes, it sounds natural.

It has several very normal features of Brazilian Portuguese:

• a não ser que
  • subjunctive
• vou terminar for the future
• rascunho for draft
• à noite as a time expression

A Brazilian speaker would understand it immediately, and it sounds like a normal sentence someone might say when talking about writing or planning work.