Eu preciso comprar farinha para fazer o bolo.

Yes. In Brazilian Portuguese, subject pronouns are often omitted when the verb already makes the subject clear. So Preciso comprar farinha para fazer o bolo is very natural.

You would keep eu if you want emphasis, contrast, or extra clarity:

• Eu preciso comprar farinha, mas ela não precisa.

Preciso is the eu form of the verb precisar in the present tense.

• precisar = to need
• eu preciso = I need

So the sentence starts with the fully conjugated verb for I.

Because after a conjugated verb like preciso, the next verb usually stays in the infinitive.

• Eu preciso comprar = I need to buy

This is very similar to English need to buy, where need is the main conjugated verb and buy stays in its base form.

This is a very common learner question.

In Brazilian Portuguese, precisar usually works like this:

• precisar + infinitive
  Eu preciso comprar farinha.
  = I need to buy flour.

• precisar de + noun
  Eu preciso de farinha.
  = I need flour.

So in your sentence, because comprar is a verb, de is not used.

Because farinha is being used in a general, non-specific sense, like flour in English.

• comprar farinha = buy flour
• comprar a farinha = buy the flour

If you say a farinha, it usually means a specific flour already known in the conversation.

By itself, farinha can mean flour in a general sense, but the exact type depends on context.

In a cake sentence, many people will understand it as wheat flour, but if you want to be specific, you can say:

• farinha de trigo = wheat flour
• farinha de milho = corn flour / cornmeal
• farinha de mandioca = cassava flour

So farinha alone is natural, but sometimes more detail is needed.

Para shows purpose: for or in order to.

So:

• para fazer o bolo = to make the cake / in order to make the cake

It explains why the speaker needs to buy flour.

In informal Brazilian Portuguese, people very often say pra instead of para:

• Eu preciso comprar farinha pra fazer o bolo.

That is extremely common in speech.

O bolo means the cake, so it sounds like a specific cake is already in mind.

• fazer o bolo = make the cake
• fazer um bolo = make a cake

Portuguese often uses the definite article in places where English might be less specific. In this sentence, o bolo sounds very natural if the speaker is talking about a particular cake they plan to make.

Yes. Portuguese allows some flexibility.

For example, you can say:

• Eu preciso comprar farinha para fazer o bolo.
• Para fazer o bolo, eu preciso comprar farinha.

Both are correct. The second version puts more emphasis on the purpose: to make the cake.

Yes. Eu tenho que comprar farinha para fazer o bolo is also natural.

The difference is roughly:

• preciso = I need
• tenho que = I have to

In everyday Brazilian Portuguese, both are common. Tenho que can sound a little more forceful or practical, while preciso is slightly closer to need.

A rough English-style approximation is:

eh-oo preh-SEE-zoo kohm-PRAHR fah-REEN-yah PAH-rah fah-ZEHR oo BOH-loo

A few useful pronunciation notes:

• Eu often sounds a bit like eh-oo or ew
• preciso has stress on si: preh-SEE-zoo
• farinha has the nh sound, like ny in canyon
• unstressed final o often sounds like u in Brazilian Portuguese, so bolo can sound closer to bolu
• in natural speech, para is often reduced to pra

So in everyday speech, you may hear something closer to:

Eu preciso comprar farinha pra fazer o bolo.