Eu lavo as mãos antes de comer.

Yes. In Brazilian Portuguese, subject pronouns are often omitted when the verb form already makes the subject clear.

• lavo clearly means I wash
• so Eu lavo... and Lavo... are both possible

Including eu can add emphasis, contrast, or clarity. In a neutral sentence, dropping it is very common.

Lavo is the 1st person singular form of lavar in the present tense.

The present tense of lavar begins like this:

• eu lavo = I wash
• você/ele/ela lava = you/he/she washes
• nós lavamos = we wash

So lavo is used because the subject is eu.

As for lavo-me: in Brazilian Portuguese, that sounds formal or unnatural in everyday speech. For this sentence, Eu lavo as mãos is the normal way to say it.

With body parts, Portuguese often uses the definite article rather than a possessive adjective when possession is obvious.

So:

• Eu lavo as mãos = literally I wash the hands
• but it naturally means I wash my hands

Because the subject is I, it is already understood that they are my hands.

You can say minhas mãos, but that usually adds emphasis, contrast, or a special nuance, for example:

• Eu lavo minhas mãos, não as suas.
  I wash my hands, not yours.

Because mão is a feminine noun, even though it ends in -o, which is unusual for learners.

• a mão = the hand
• as mãos = the hands

So the article must agree with the noun:

• singular: a
• plural: as

This is just one of those nouns whose gender must be memorized.

Because the normal idea is that you wash both hands before eating.

So:

• Eu lavo a mão = I wash my hand / one hand
• Eu lavo as mãos = I wash my hands

In everyday language, the plural is the natural choice here.

Because after antes, Portuguese normally uses de before a verb.

So the pattern is:

• antes de + infinitive

Examples:

• antes de comer = before eating
• antes de sair = before leaving
• antes de dormir = before sleeping

So antes comer would sound incomplete or incorrect here.

After a preposition like de, Portuguese usually uses the infinitive form of the verb.

That is why you get:

• antes de comer = before eating

not:

• antes de como
• antes de comendo

The infinitive is the standard form after antes de when the subject is general or the same as the main subject.

Yes, but it means something slightly more explicit.

• antes de comer = before eating
• antes de eu comer = before I eat

The shorter version is very common when the subject is understood to be the same person as in the main clause.
The longer version sounds more explicit and can be used for emphasis or clarity.

In your sentence, antes de comer is the most natural choice.

Usually, the present tense in Portuguese can do both, depending on context.

So Eu lavo as mãos antes de comer can mean:

• a habit or routine: I wash my hands before eating
• something true in general: I do that before I eat

Without extra context, it most naturally sounds like a habitual action or general statement.

If you wanted to stress an action happening right now, Portuguese would often use:

• Estou lavando as mãos antes de comer.

But for routines and general truths, the simple present is perfect.

Yes. That word order is completely natural.

Both are correct:

• Eu lavo as mãos antes de comer.
• Antes de comer, eu lavo as mãos.

The difference is mostly one of emphasis:

• starting with Eu lavo... focuses first on the action
• starting with Antes de comer... highlights the time expression first

Both sound normal in Brazilian Portuguese.