Eu lavo o cabelo à noite.

Yes. In Brazilian Portuguese, eu can often be omitted because the verb form already shows the subject.

• Eu lavo o cabelo à noite.
• Lavo o cabelo à noite.

Both are natural. Including eu can add emphasis, clarity, or contrast, but it is not required in a simple sentence like this.

Portuguese often uses the definite article with body parts and things closely connected to the person, where English usually uses a possessive.

So Portuguese commonly says:

• Eu lavo o cabelo.

rather than:

• Eu lavo meu cabelo.

Both can be understood, but o cabelo is the more natural default in many contexts. English prefers my hair, but Portuguese does not need the possessive here.

Lavo is the first person singular present tense of the verb lavar, meaning to wash.

Conjugation in the present:

• eu lavo = I wash
• você lava = you wash
• ele/ela lava = he/she washes
• nós lavamos = we wash
• vocês lavam = you all wash
• eles/elas lavam = they wash

So eu lavo means I wash.

In standard Portuguese, Eu lavo cabelo à noite sounds incomplete or unnatural in this context.

The normal form is:

• Eu lavo o cabelo à noite.

Portuguese usually needs the article with nouns like this, especially when referring to something specific or understood, such as your hair. Leaving out o would not sound natural in everyday speech here.

À noite is a fixed time expression meaning at night or in the evening/at night, depending on context.

The à is a contraction of:

• a
  • a

This happens because the expression historically includes the preposition a plus the feminine article a before noite.

So:

• à noite = at night

You usually need the full expression. Saying just noite would not work in this sentence.

They are different.

• à noite with a grave accent means at night
• a noite without the accent is just the night as a noun phrase, depending on the sentence

Examples:

• Eu estudo à noite. = I study at night.
• A noite está bonita. = The night is beautiful.

So in your sentence, the accent is necessary because it is the time expression à noite.

Yes. In Brazilian Portuguese, de noite is also common and natural in speech.

• Eu lavo o cabelo à noite.
• Eu lavo o cabelo de noite.

Both are used. In many situations, à noite may sound a little more neutral or formal, while de noite can sound a bit more conversational, but both are standard and common in Brazil.

In this sentence, the simple present lavo most naturally expresses a habit or routine:

• Eu lavo o cabelo à noite. = I wash my hair at night / I wash my hair at night as a habit.

If you want to emphasize an action happening right now, Brazilian Portuguese often uses:

• Estou lavando o cabelo. = I am washing my hair.

If you want to talk about tonight specifically, context matters. Often Portuguese would make that clearer with other words, such as:

• Vou lavar o cabelo à noite. = I’m going to wash my hair tonight.

In Portuguese, cabelo can be singular when referring to the hair on your head as a whole.

So:

• lavar o cabelo = wash one’s hair

This is completely normal. Portuguese can also use the plural cabelos, but that usually sounds more like individual hairs or can be used in certain stylistic contexts. For the everyday idea of washing your hair, o cabelo is the usual choice.

Yes. Portuguese word order is flexible enough that you can move the time expression for emphasis.

All of these are possible:

• Eu lavo o cabelo à noite.
• À noite, eu lavo o cabelo.
• Eu, à noite, lavo o cabelo. — more marked or emphatic

The first one is the most neutral and straightforward.

A rough Brazilian Portuguese pronunciation guide:

• Eu ≈ eh-oo blended quickly
• lavo ≈ LAH-voo
• o before cabelo is often pronounced like oo
• cabelo ≈ kah-BEH-loo
• à noite ≈ a NOY-chee in many Brazilian accents

A natural flow can sound roughly like:

• eh-u LAH-voo oo kah-BEH-loo a NOY-chee

A few useful pronunciation notes:

• In Brazil, unstressed final -o often sounds like oo
• noi in noite sounds like noy
• Final -te in many Brazilian accents sounds like chee

No, it is not wrong. It is grammatical and understandable.

But in ordinary Portuguese, Eu lavo o cabelo à noite is usually more natural. Using meu cabelo can sound more explicit or emphatic, for example if you are contrasting your hair with someone else’s:

• Eu lavo meu cabelo, não o dela. = I wash my hair, not hers.

So o cabelo is usually the best default here.

Not in this sentence.

Portuguese can use reflexive forms with personal care actions in some cases, but here the normal sentence is simply:

• Eu lavo o cabelo.

You may also hear structures like:

• Eu lavo o cabelo.
• Eu vou lavar o cabelo.

A reflexive pronoun is not necessary here. For an English speaker, the important thing is that Portuguese does not require a structure like I wash myself the hair. Just use lavar o cabelo.