Learn Hardcore Portuguese (Brazil): Eu não sei qual curso escolher, porque tenho muitas ideias. - I don’t know which course to choose, because I have many ideas.

qual curso – slightly more neutral or careful; very common in both speech and writing.
que curso – also correct, and very common in everyday speech. Some people feel it sounds a bit more informal.

In practice:

Eu não sei qual curso escolher.
Eu não sei que curso escolher.

both mean “I don’t know which course to choose.” There’s no real change in meaning; it’s mostly a style/feel difference.

Why is it qual curso escolher (infinitive) instead of something like qual curso devo escolher or qual curso eu escolho?

Portuguese often uses an infinitive after a question word (qual, que, quem, onde, como, quando) when the subject of the two verbs is the same.

Main verb: (Eu) não sei – subject eu
Infinitive clause: (eu) escolher – same subject, eu

So instead of repeating the subject, you use an infinitive:

Eu não sei qual curso escolher.
= literally “I don’t know which course to choose.”

You could also say:

Eu não sei qual curso devo escolher.
Eu não sei qual curso eu escolho.

These are grammatical, but qual curso escolher with the infinitive is more compact and very natural.

Why is there no article before curso? Why not qual o curso escolher?

Portuguese can use either structure:

qual curso escolher
qual o curso escolher

In this specific sentence, qual curso escolher (without article) is more natural and more common, because:

With qual + singular countable noun, omitting the article is very frequent when you mean “which … (out of several options).”
- Não sei qual filme ver.
- Não sei qual livro comprar.
qual o curso can sound a bit heavier or more specific, and is more natural in other contexts, such as:
- Qual é o curso que você quer fazer?
  (“Which is the course you want to take?”)

So qual curso escolher is the default choice here, but qual o curso escolher is not ungrammatical; it just sounds less idiomatic in this exact sentence.

Why is the negation não placed before sei? Could I say Eu sei não?

In Portuguese, não normally comes before the main verb in simple sentences:

Eu não sei.
Eu não entendo.
Ela não gosta.

Eu sei não is wrong in standard Portuguese. The usual order is:

subject (optional) + não + verb

So:

Eu não sei qual curso escolher. ✅
Não sei qual curso escolher. ✅ (dropping eu)
Eu sei não qual curso escolher. ❌

Why is it sei and not conheço for “I don’t know”?

Portuguese has two main verbs for “to know”:

saber – to know facts, information, how to do something
- Eu sei a resposta. – I know the answer.
- Eu sei dirigir. – I know how to drive.
- Eu não sei qual curso escolher. – I don’t know which course to choose.
conhecer – to know people, places, things (be familiar with)
- Eu conheço o João. – I know João.
- Eu conheço São Paulo. – I know (have been to) São Paulo.
- Você conhece este curso? – Do you know / are you familiar with this course?

Here, you’re talking about knowing a piece of information (which course to choose), so saber is required:

Eu não sei qual curso escolher. ✅
Eu não conheço qual curso escolher. ❌ (ungrammatical in this sense)

Can I drop the subject eu and just say Não sei qual curso escolher, porque tenho muitas ideias?

Yes. In Portuguese, subject pronouns are often optional, because the verb ending usually tells you who the subject is.

Eu não sei qual curso escolher, porque tenho muitas ideias.
Não sei qual curso escolher, porque tenho muitas ideias.

Both are correct. Differences in feel:

Including eu can add a bit of emphasis on “I” or sound a bit more formal/careful.
Omitting eu sounds very natural in everyday speech and in writing.

In conversation, many people would naturally say the version without eu.

Why is there a comma before porque? Could I also write it without a comma?

You can write the sentence with or without the comma before porque, with a slight nuance:

Eu não sei qual curso escolher, porque tenho muitas ideias.
With comma: the clause with porque sounds more like an explanation/additional comment:
“I don’t know which course to choose, because I have lots of ideas, you see.”
Eu não sei qual curso escolher porque tenho muitas ideias.
Without comma: the porque clause is more tightly connected as a direct cause of not knowing.

In everyday usage, both forms are common. Many writers would omit the comma in a straightforward causal sentence, but keeping it is not wrong here and is often used to give a more explanatory tone.

Why is porque one word here and not por que / por quê / porquê?

Portuguese has four forms:

porque – conjunction meaning “because” (and also sometimes “that” in some explanations)
- Não fui porque estava doente.
- Eu não sei qual curso escolher, porque tenho muitas ideias.
por que – “why” in direct or indirect questions, or “for which”
- Por que você está triste? – Why are you sad?
- Quero saber por que você saiu. – I want to know why you left.
por quê – “why” at the end of a sentence
- Você saiu por quê?
porquê – a noun meaning “reason”
- Não entendo o porquê disso. – I don’t understand the reason for this.

In the sentence given, porque is a conjunction meaning “because”, so it must be written as one word.

Why is it tenho muitas ideias and not something like sou com muitas ideias?

To express “I have a lot of ideas”, Portuguese normally uses ter:

Eu tenho muitas ideias. – literally “I have many ideas.”

Some key points:

ter is the usual verb for possession and also for many abstract things:
- Tenho dúvidas. – I have doubts.
- Tenho tempo. – I have time.
- Tenho medo. – I’m afraid (literally “I have fear”).
ser is used for characteristics/identity (to be), not for “having”:
- Eu sou criativo. – I am creative.
- Eu sou estudante. – I am a student.

So sou com muitas ideias is ungrammatical. For this meaning you need ter:

(Eu) tenho muitas ideias. ✅

Why is it muitas ideias (with -as) and not muitos ideias?

In Portuguese, determiners and adjectives agree in gender and number with the noun:

ideia is feminine singular: a ideia
ideias is feminine plural: as ideias

So:

singular:
- muita ideia – much idea / a lot of idea (not very natural, but correct grammatically)
plural:
- muitas ideias – many ideas / lots of ideas

Because ideias is feminine plural, the quantifier must also be feminine plural:

muitas ideias ✅
muitos ideias ❌ (wrong gender agreement)
muitos ideiasS ❌ (double plural is impossible; only the noun takes -s, not the quantifier’s stem, beyond its own plural ending)

Could I say porque estou com muitas ideias instead of porque tenho muitas ideias?

Yes, you could say:

Eu não sei qual curso escolher, porque estou com muitas ideias.

Both are grammatical, but there’s a nuance:

tenho muitas ideias – neutral; just states the fact that you have many ideas in general or at this moment.
estou com muitas ideias – more colloquial; emphasizes a current state or phase, roughly “I’m (currently) full of ideas.”

For this sentence, tenho muitas ideias is the most straightforward and common way to say “I have a lot of ideas.”

Can I change the order and start the sentence with Porque?

Yes. You can place the porque clause first:

Porque tenho muitas ideias, eu não sei qual curso escolher.

This is perfectly correct and sounds natural. Differences:

Original order:
- Eu não sei qual curso escolher, porque tenho muitas ideias.
  Starts with the problem, then gives the reason.
Reversed order:
- Porque tenho muitas ideias, eu não sei qual curso escolher.
  Starts with the reason, then states the consequence.

In spoken Portuguese, the original order (reason after the main clause) is more common, but both are used.

Does curso here mean “class” or “major”? How is curso used in Brazilian Portuguese?

In Brazilian Portuguese, curso is a flexible word. Common meanings:

A course / a program of study (what you study at college)
- Que curso você faz? – What’s your major? / What course are you doing?
- Eu faço o curso de Engenharia. – I’m doing an Engineering degree.
A shorter course, training, or class series
- Fiz um curso de inglês. – I took an English course.
- Vou fazer um curso de programação. – I’m going to take a programming course.

In the sentence Eu não sei qual curso escolher, curso is most naturally understood as a course of study / major / training course, depending on context. It’s not used for a single class session; for that, people usually say aula.