Se a primeira fatia estiver boa, levo outra para a minha mãe.

Because Portuguese often uses the future subjunctive after se when talking about a possible future condition.

So in:

Se a primeira fatia estiver boa...

the idea is If the first slice turns out to be good...

This is different from a general fact like:

Se a fatia está boa, como-a.
= If the slice is good, I eat it. / When the slice is good, I eat it.

A very common pattern is:

• Se eu tiver tempo, vou. = If I have time, I’ll go.
• Se ele puder, vem. = If he can, he’ll come.
• Se estiver boa, levo outra. = If it’s good, I’ll take another.

So estiver is used because the speaker is talking about something not yet confirmed in the future.

Estiver is the future subjunctive of estar.

Here is the full form:

• eu estiver
• tu estiveres
• ele/ela/você estiver
• nós estivermos
• vós estiverdes
• eles/elas/vocês estiverem

This tense is very common in Portuguese, especially after words like:

• se = if
• quando = when
• logo que = as soon as
• assim que = as soon as

Examples:

• Quando ele chegar, avisamos.
• Se eu souber, digo-te.
• Assim que estiver pronto, mando.

English does not really have a direct equivalent tense, so it can feel unfamiliar at first.

In Portuguese, the present indicative is often used to talk about the future, especially when the future meaning is already clear from the context.

So:

Se a primeira fatia estiver boa, levo outra...

literally looks like If the first slice is good, I take another..., but it naturally means:

If the first slice is good, I’ll take another...

This is very normal in Portuguese.

You could also say:

Se a primeira fatia estiver boa, levarei outra para a minha mãe.

That uses the simple future levarei, but in everyday speech levo is usually more natural and common.

That is just one of the major grammar differences between English and Portuguese.

In English, after if, we often use a present form for future meaning:

• If it is good, I’ll take another.

In Portuguese, when the condition refers to the future and is uncertain, the usual pattern is:

• se + future subjunctive
• then a main clause often in the present or future

So:

• Se estiver boa, levo outra.
• Se ele vier, falamos com ele.
• Se tiveres tempo, ajuda-me.

For an English speaker, this is one of the most important sentence patterns to get used to.

Fatia usually means slice or piece cut from something, especially food.

In this sentence, a primeira fatia is most naturally:

• the first slice of cake
• the first slice of pizza
• the first slice of pie
• the first slice of some other food served in slices

It is not usually used for every kind of piece. For example:

• fatia de bolo = slice of cake
• fatia de pizza = slice of pizza
• fatia de pão = slice of bread

So yes, slice is the best translation here.

Outra means another one or a different one.

In:

levo outra para a minha mãe

the word outra stands in for outra fatia.

So the full idea is:

I’ll take another slice for my mother.

Portuguese often leaves out the noun when it is obvious:

• Quero este bolo e outro. = I want this cake and another one.
• Compro uma garrafa e levo outra. = I buy one bottle and take another.

Here, outra is feminine singular because it refers to fatia, which is feminine:

• uma fatia
• outra

Portuguese usually uses the definite article with ordinal numbers in this kind of context.

So:

• a primeira fatia = the first slice
• o primeiro dia = the first day
• a segunda vez = the second time

Leaving out the article would sound incomplete or unnatural in most normal sentences.

In European Portuguese, using the article before possessives is very common and usually the most natural choice:

• a minha mãe
• o meu pai
• a minha irmã

So:

para a minha mãe = for my mother

In Brazilian Portuguese, people often omit the article more freely:

• para minha mãe

But in Portugal, para a minha mãe is the safer and more natural form for everyday speech.

Not in this sentence, because levar here is being used with the idea take something for someone, not take something to someone as a direct recipient structure.

Compare:

• Levo outra para a minha mãe.
  = I’ll take another one for my mother.

This suggests the speaker is taking it with the purpose of giving it to her.

But you can also have:

• Levo outra à minha mãe.

This would sound more like I take another one to my mother, with stronger focus on destination/recipient. Depending on context, it may be possible, but para a minha mãe is very natural when the meaning is for my mother.

So para highlights intended beneficiary more clearly.

Because Portuguese often leaves out subject pronouns when the subject is understood from the context.

In:

Se a primeira fatia estiver boa...

the subject is already explicitly stated: a primeira fatia.

So there is no need for anything like English it.

Also, Portuguese adjectives agree with the noun:

• fatia is feminine singular
• so boa is feminine singular too

That agreement helps confirm what the adjective refers to.

Because boa must agree with fatia, which is a feminine singular noun.

• fatia = feminine singular
• boa = feminine singular form of bom

Agreement examples:

• o bolo está bom = the cake is good
• a fatia está boa = the slice is good
• as fatias estão boas = the slices are good

This kind of adjective agreement is very important in Portuguese.

Yes. Boa can be translated in different ways depending on context:

• good
• nice
• tasty
• okay
• fine

With food, boa often means something like:

• good
• tasty
• nice

So the exact English wording depends on the situation. If someone is testing food, boa might naturally mean good or tasty.

Yes, the grammar is natural. The sentence is perfectly plausible.

It has a very typical Portuguese structure:

• Se + future subjunctive
• main clause in present with future meaning

That said, in real life, a speaker might also say slightly different versions depending on style and context, for example:

• Se a primeira fatia for boa, levo outra para a minha mãe.
• Se esta fatia estiver boa, levo outra para a minha mãe.
• Se a primeira estiver boa, levo outra para a minha mãe.

But the original sentence is grammatically natural and useful for learning.

Both can appear after se, but they are based on different verbs:

• estar → estiver
• ser → for

In many food-related contexts, estar boa is very natural because it refers to a condition or state:

• A sopa está boa. = The soup is good.

You may also hear ser boa in other contexts, depending on the meaning:

• Se a pizza for boa, voltamos aqui.

In practice, with food, both can sometimes occur, but estar boa often feels especially natural when talking about how something tastes at that moment.

Yes, to some extent.

The original order is very normal:

Se a primeira fatia estiver boa, levo outra para a minha mãe.

You could also say:

Levo outra para a minha mãe se a primeira fatia estiver boa.

That means the same thing, but putting the se clause first is very common when you want to present the condition first.

So both are correct; the original is just a very natural, clear structure.