Eu tenho que tirar o chapéu quando entro no restaurante.

Not necessarily.

Portuguese often leaves out subject pronouns because the verb ending already tells you who the subject is. Here, tenho already means I have, so:

• Eu tenho que tirar o chapéu...
• Tenho que tirar o chapéu...

Both are correct.

Using eu can add emphasis, clarity, or contrast. For example, if you want to stress that I have to do it, not someone else, eu is more likely to appear.

Yes. Ter que + infinitive is a very common way to say have to / must in Brazilian Portuguese.

So:

• tenho que tirar = I have to take off

A close variant is ter de + infinitive:

• tenho de tirar

That also means I have to take off, but ter que is especially common in everyday Brazilian speech.

Yes, and it would sound natural.

But there is a small nuance:

• tenho que = obligation, rule, requirement
• preciso = need to, necessity

So:

• Tenho que tirar o chapéu suggests there is a rule or expectation.
• Preciso tirar o chapéu can sound a bit more like a practical need.

In many real situations, though, the two are very close.

Portuguese often uses the definite article with clothing, body parts, and personal items when the owner is obvious from the context.

So tirar o chapéu often naturally means take off my hat in this kind of sentence.

This is very common in Portuguese:

• Lavei as mãos = I washed my hands
• Quebrei o braço = I broke my arm
• Tirei o casaco = I took off my coat

You can say meu chapéu, but it usually adds emphasis or contrast:

• Tenho que tirar meu chapéu, não o seu.

It can be both.

Literally, it means to take off one’s hat.

But idiomatically, tirar o chapéu para alguém means something like take my hat off to someone or I really admire/respect that person.

Example:

• Tiro o chapéu para ela. = I take my hat off to her.

In your sentence, because of quando entro no restaurante, the literal meaning is the natural one.

Both are correct.

Just like at the beginning of the sentence, Portuguese often drops the subject pronoun when the verb form already makes it clear. Entro already means I enter / I go in.

So both of these work:

• quando entro no restaurante
• quando eu entro no restaurante

The version without eu is very natural and common.

Because this sentence describes a general rule, habit, or repeated situation.

So quando entro no restaurante means something like:

• when I enter the restaurant
• whenever I enter the restaurant

Portuguese uses the present tense here for habitual or general situations.

If you were talking about one specific future occasion, Portuguese would usually use the future subjunctive:

• quando eu entrar no restaurante

That means when I enter the restaurant in the sense of a future event.

So the contrast is:

• quando entro = whenever I enter / as a habit
• quando eu entrar = when I enter in the future

Because no is a contraction.

In Portuguese:

• em + o = no

So:

• no restaurante = in the restaurant / at the restaurant

This kind of contraction is extremely common:

• na = em + a
• nos = em + os
• nas = em + as

So em o restaurante would sound wrong in normal Portuguese.

Because noun gender in Portuguese is not determined only by the final letter.

Restaurante is a masculine noun:

• o restaurante

Many nouns ending in -e can be either masculine or feminine, so you usually need to learn the article together with the noun.

For example:

• o restaurante
• o leite
• a noite
• a parede

So it is best to memorize vocabulary with the article: o restaurante, not just restaurante.

The accent mark in chapéu shows the stressed syllable and helps indicate vowel quality.

The stress falls on pé:

• cha-PÉU

A rough English approximation is something like sha-PEH-oo, said smoothly as one word, though the real Portuguese sound is not exactly the same as English sounds.

A few pronunciation notes:

• ch in Brazilian Portuguese sounds like sh
• é is an open eh sound
• eu here forms a diphthong, not two fully separate syllables

So chapéu has two syllables: cha-péu.

Yes, absolutely.

That version is perfectly correct and very natural. It just puts the time clause first:

• Quando entro no restaurante, eu tenho que tirar o chapéu.

Both orders work:

• Eu tenho que tirar o chapéu quando entro no restaurante.
• Quando entro no restaurante, eu tenho que tirar o chapéu.

The difference is mainly one of emphasis and style, not grammar.

Grammatically, yes.

Culturally, it may sound a little formal, old-fashioned, or unusual because wearing a hat into a restaurant is not a very common everyday situation in modern Brazil.

But the sentence itself is completely understandable and correct. If the context is a formal place, a rule, an etiquette lesson, or a historical setting, it sounds fine.

So:

• grammatically natural: yes
• common everyday situation: not especially