Learn Hardcore Portuguese (Portugal): Eu não quero entrar no supermercado porque está cheio. - I don't want to enter the supermarket because it is full.

Yes. In Portuguese (including European Portuguese) the subject pronoun is often dropped because the verb ending (-o in quero) already shows that the subject is eu.

Eu não quero entrar… – correct, a bit more explicit or emphatic.
Não quero entrar… – also correct, very natural in everyday speech.

You usually keep eu for emphasis (e.g. contrasting with someone else: Eu não quero, mas ela quer.).

Why is não placed before quero and not somewhere else?

The basic rule is: não goes directly before the conjugated verb.

Here, the conjugated verb is quero (from querer), so you say:

Eu não quero entrar…

If there are other verbs in the infinitive or a past participle, não still goes before the main finite verb:

Eu não quero entrar.
Eu não tenho querido entrar.
Eu não vou entrar.

You don’t put não at the end like in English (no “I want not” or “I enter not” structure).

Why is it entrar no supermercado and not entrar o supermercado?

The verb entrar normally takes the preposition em (to enter / go into somewhere):

entrar em casa – to go into the house
entrar em Lisboa – to enter Lisbon

When em comes before the definite article o (the), they contract to no:

em + o supermercado = no supermercado

So entrar no supermercado literally means “to enter in the supermarket”, which in English we simply say as “enter the supermarket / go into the supermarket.”

You cannot say entrar o supermercado; you need em (→ no).

What exactly is no here? Why not em o supermercado written separately?

In Portuguese, many prepositions + definite articles contract into a single word.

For em (in / on / at) you get:

em + o = no (masc. singular)
em + a = na (fem. singular)
em + os = nos (masc. plural)
em + as = nas (fem. plural)

So:

no supermercado = em o supermercado = in the / into the supermarket
na loja = em a loja = in the / into the shop

You normally use the contracted forms (no, na, nos, nas) in standard Portuguese.

Could I say entrar ao supermercado or ir ao supermercado instead? What’s the difference?

Two things are happening here: the verb and the preposition.

entrar em is the standard pattern → entrar no supermercado (to enter / go into the supermarket).
- entrar ao supermercado is not idiomatic; you would not say that.
ir a means to go to (destination), so:
- ir ao supermercado (a + o = ao) → to go to the supermarket (not yet inside; just going there).

So:

Não quero entrar no supermercado – I don’t want to go into / inside the supermarket.
Não quero ir ao supermercado – I don’t want to go to the supermarket (at all).

Both are correct but describe slightly different actions.

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

In this sentence porque means “because” and it introduces a reason clause. In that case, it’s always written as one word: porque.

The other spellings are used in different contexts:

por que – usually in questions (especially in Brazilian Portuguese), similar to “why / for what reason”.
- Por que não queres entrar? – Why don’t you want to go in?
porquê – a noun meaning “the reason”.
- Não entendo o porquê. – I don’t understand the reason.
por quê – in Brazilian Portuguese, por que written at the end of a question:
- Não queres entrar por quê? – Why don’t you want to go in?

In your sentence, we’re explaining the reason, so we must use porque = because.

In porque está cheio, what is the subject of está? Why is there no pronoun?

The implied subject of está is o supermercado. Portuguese often drops the subject when it is clear from context, especially when it was just mentioned:

(O supermercado) está cheio. → Está cheio.

So the full version would be:

Eu não quero entrar no supermercado porque o supermercado está cheio.

That’s grammatical but sounds repetitive. Natural Portuguese drops the second o supermercado (and also the pronoun ele), because it’s obvious what está cheio refers to.

Why is it cheio and not cheia? How does the agreement work?

Adjectives in Portuguese agree in gender and number with the noun they describe.

supermercado is masculine singular (o supermercado).
So the adjective must also be masculine singular: cheio.

If the noun changed, the adjective would change too:

A loja está cheia. – The shop is full/crowded. (feminine singular)
Os supermercados estão cheios. – The supermarkets are full. (masc. plural)
As lojas estão cheias. – The shops are full. (fem. plural)

So cheio matches o supermercado in gender and number.

Does estar cheio here mean “full” like full of products, or “crowded” with people?

In this context, estar cheio normally means “crowded / very busy” (full of people), because that’s the usual reason someone wouldn’t want to go in.

estar cheio can mean:

full (of people) – crowded:
- O supermercado está cheio. – The supermarket is packed / crowded.
full (of something) – containing a lot of something:
- O saco está cheio de compras. – The bag is full of shopping.

Context decides which sense you mean. Here, with entrar no supermercado, the natural reading is “crowded”.

Could I move porque está cheio to the beginning: Porque está cheio, eu não quero entrar no supermercado?

Yes, that word order is perfectly correct:

Eu não quero entrar no supermercado porque está cheio.
Porque está cheio, eu não quero entrar no supermercado.

Both are natural. Starting with Porque está cheio puts a bit more emphasis on the reason and is slightly more typical of written language or careful speech, but it’s fine in conversation too.

Note: Unlike some very traditional advice in English about not starting a sentence with “Because…”, in Portuguese beginning with Porque is completely normal.

Do I need a comma before porque in this sentence?

No comma is needed here, and the most standard writing is:

Eu não quero entrar no supermercado porque está cheio.

In Portuguese, you generally do not put a comma before porque when it simply introduces a reason clause right after the main clause. A comma can appear in special cases (for extra emphasis or in more complex sentences), but in a simple sentence like this, you normally write it without a comma.