Para o bolo, preciso de farinha e de fermento.

Because Para o bolo sets the context first: it means for the cake. Starting with it is a natural way to frame the sentence, like saying:

• For the cake, I need flour and baking powder/yeast.

In Portuguese, this kind of fronting is very common when you want to make the purpose or topic clear first. You could also say:

• Preciso de farinha e de fermento para o bolo.

That is also correct, but it sounds a little less like as for the cake...

It actually is two words here: para + o.

• para = for
• o = the (masculine singular)
• bolo = cake

So para o bolo literally means for the cake.

Unlike some other combinations in Portuguese, para + o does not normally contract in standard writing into one word. In speech, especially informal speech, you may hear it reduced, but in writing para o is the normal form.

Because the verb precisar normally takes the preposition de when it means to need.

So:

• precisar de alguma coisa = to need something

Examples:

• Preciso de água. = I need water.
• Precisamos de ajuda. = We need help.
• Ela precisa de tempo. = She needs time.

For an English speaker, this is important because English says need something, but Portuguese usually says need of something with de.

Portuguese often repeats the preposition before each item in a list, especially in careful or natural phrasing.

So:

• preciso de farinha e de fermento

is very natural.

You may also hear:

• preciso de farinha e fermento

and that can be understood, but repeating de is very common and often sounds more complete or more elegant.

So the repeated de is not strange — it is normal Portuguese structure.

Because here they are being used in a general, indefinite sense: some flour and some baking powder/yeast, not the flour or the baking powder/yeast.

Compare:

• Preciso de farinha. = I need flour.
• Preciso da farinha. = I need the flour.

The second one refers to a specific flour, perhaps already mentioned.

This is very similar to English, where you also often say I need flour, not I need the flour, unless you mean a specific one.

Here, preciso is a verb form: the 1st person singular present of precisar.

• eu preciso = I need

Full conjugation in the present:

• eu preciso
• tu precisas
• ele/ela precisa
• nós precisamos
• vós precisais (rare in modern speech)
• eles/elas precisam

Be careful: preciso can also be an adjective in other contexts, meaning precise or accurate.

For example:

• um valor preciso = an exact value

But in your sentence, it is definitely the verb I need.

Yes, absolutely.

Portuguese often drops subject pronouns when they are clear from the verb form, so:

• Preciso de farinha e de fermento.
• Eu preciso de farinha e de fermento.

both mean the same thing.

In Portuguese, leaving out eu is often the more neutral choice. Including eu can add emphasis, contrast, or clarity.

For example:

• Eu preciso de farinha; tu precisas de açúcar.

Here eu and tu are useful for contrast.

Because bolo is a masculine noun in Portuguese, so it takes the masculine singular article o:

• o bolo = the cake

Grammatical gender in Portuguese is something you usually have to learn with each noun. It does not always match anything logical in English.

Here the important thing is simply:

• bolo → masculine
• therefore o bolo

In a cooking sentence like this, fermento usually refers to a raising agent.

In Portugal, the exact meaning depends on context:

• fermento em pó = baking powder
• fermento de padeiro = baker’s yeast / fresh or dry yeast for bread

With bolo (cake), many learners will understand fermento as shorthand for fermento em pó, since that is what cakes commonly use.

So context matters. If you want to be completely specific, you can say:

• Preciso de farinha e de fermento em pó.

Yes. It is a complete sentence.

Portuguese is a pro-drop language, which means subject pronouns are often omitted when the verb ending already shows who the subject is.

In:

• preciso

the ending -o shows that the subject is I.

So Preciso de farinha e de fermento is a complete and perfectly natural sentence.

In European Portuguese, several sounds are reduced in normal speech.

A few useful points:

• para is often pronounced in a reduced way, something like p'ra
• de is often very short
• unstressed vowels are often weakened in European Portuguese

So in natural speech, the sentence may sound more compressed than its spelling suggests.

Very roughly, you might hear something like:

• P'ró bolo, preciso de farinha e de fermento.

That is just a rough learner-friendly approximation, not a formal spelling change.

The main thing to know is that spoken European Portuguese often sounds more reduced than what you see written.