Se faltar farinha ou fermento, não faço o bolo hoje.

Because after se to talk about a possible future condition, Portuguese normally uses the future subjunctive.

So:

• Se faltar farinha... = If flour is missing / If we don't have flour...
• Se falta farinha... would sound more like a habitual or general situation, not this specific future possibility.

A useful pattern is:

• Se + future subjunctive, main clause

For example:

• Se chover, fico em casa. = If it rains, I’m staying home.
• Se houver tempo, faço isso amanhã. = If there’s time, I’ll do that tomorrow.

With regular verbs, the future subjunctive often looks the same as the infinitive, which is why faltar does not change here.

Here faltar means to be missing, to be lacking, or for there not to be enough / any.

So faltar farinha is not literally to miss flour in the emotional English sense. It means:

• flour is missing
• there isn’t any flour
• we’re out of flour

This verb is very common in Portuguese for things that are absent or unavailable:

• Falta açúcar. = There’s no sugar / We’re out of sugar.
• Falta dinheiro. = There isn’t enough money.
• Ele faltou à aula. = He missed class / He was absent from class.

Because ou presents the items as alternatives: flour or baking powder/yeast.

The idea is:

• if flour is missing
• or if baking powder/yeast is missing

Since each item is singular on its own, the singular verb is natural here.

So Se faltar farinha ou fermento... means that the absence of either one would be enough to stop the cake from being made.

If the sentence used e instead, the plural would be more likely:

• Se faltarem a farinha e o fermento... = If both the flour and the baking powder/yeast are missing...

In normal understanding, it means either one is enough:

• if flour is missing, no cake
• if baking powder/yeast is missing, no cake

It can also logically include the situation where both are missing, because if both are missing, then at least one is missing. But the practical meaning is:

even one missing ingredient is enough to prevent making the cake.

Because here they are being used in a general, non-specific way, like ingredient names or mass nouns.

Portuguese often leaves out the article in cases like this:

• farinha = flour
• fermento = baking powder/yeast

This is similar to English, where we also often say flour rather than the flour.

If you wanted to refer to specific, already-known ingredients, you could use articles:

• Se faltar a farinha ou o fermento...

That sounds more like:

• if the flour or the baking powder/yeast is missing

So both versions are possible, but the version without articles is very natural when talking about ingredients in general.

Because Portuguese often uses the present tense to talk about a near future action, especially in everyday speech.

So:

• não faço o bolo hoje = I’m not making the cake today / I won’t make the cake today

This is very natural.

Other possibilities are:

• não farei o bolo hoje = I will not make the cake today
  
  • more formal or firm
• não vou fazer o bolo hoje = I’m not going to make the cake today
  
  • very common in speech

So não faço is not a literal present in English; it is a normal Portuguese way to express a future decision or plan.

Because o bolo usually refers to a specific cake that both speaker and listener can identify from the context.

Here it means something like:

• the cake I was going to make
• the cake for today

Portuguese uses the definite article more often than English in this kind of situation.

Compare:

• Faço o bolo hoje. = I’m making the cake today.
  
  • a specific cake
• Fazer bolo é fácil. = Making cake is easy.
  
  • more generic

So in this sentence, o bolo sounds natural because it refers to a particular planned cake.

Faço is the 1st person singular present indicative of fazer:

• eu faço = I do / I make

In this sentence:

• não faço o bolo hoje = I’m not making the cake today

It is worth noticing that fazer is not fully regular in the present tense:

• eu faço
• tu fazes
• ele/ela faz
• nós fazemos
• vocês fazem

The ç in faço is there to keep the s sound before o. Without it, c before o would sound like k.

In cooking, fermento is a general word for a leavening agent, but the exact meaning depends on context.

In Portugal, in a sentence about making a cake, fermento usually suggests:

• baking powder
  often said more fully as fermento em pó

In other contexts, it can also mean:

• yeast

So if the context is cake-making, many learners will understand fermento here as baking powder.

Because that is a very natural place for it.

Portuguese adverbs like hoje can move around, but the end position is neutral and common:

• Se faltar farinha ou fermento, não faço o bolo hoje.

You could also say:

• Hoje, não faço o bolo se faltar farinha ou fermento.
• Se faltar farinha ou fermento, hoje não faço o bolo.

These are possible, but they change the emphasis a little. Putting hoje at the end sounds smooth and straightforward.

Because the sentence begins with a conditional subordinate clause:

• Se faltar farinha ou fermento, ...

When this kind of clause comes first in Portuguese, it is normally separated from the main clause with a comma.

So the structure is:

• Se faltar farinha ou fermento,
  não faço o bolo hoje.

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

• Não faço o bolo hoje se faltar farinha ou fermento.

Both are correct, but the original punctuation is the standard choice for this word order.