Antes de jantar, eu lavo as mãos com sabonete.

Because antes de + infinitive is the normal pattern for before doing something.

• antes de jantar = before having dinner / before eating dinner
• antes do jantar = before the dinner, before the meal/event itself

Both can be correct, but they are not exactly the same structure. In your sentence, jantar is being used as a verb, so de is what you expect.

Here, jantar is a verb in the infinitive.

Portuguese uses the same form jantar for:

• the verb: to have dinner / to dine
• the noun: dinner

How do you know which one it is?

• antes de jantar → after de, this is understood as an infinitive verb
• o jantar / do jantar → the article shows it is a noun

So in this sentence, it is the verb form.

Yes, it can be omitted.

Portuguese usually does not need the subject pronoun when the verb ending already makes the subject clear:

• Antes de jantar, eu lavo as mãos com sabonete.
• Antes de jantar, lavo as mãos com sabonete.

Both are correct.

In European Portuguese, leaving out eu is very common unless you want to:

• add emphasis
• create contrast
• avoid ambiguity

So the version without eu is often the more natural everyday choice.

Lavo is the 1st person singular present indicative of lavar.

• lavar = to wash
• eu lavo = I wash

In this kind of sentence, the present tense usually expresses a habit or routine, just like in English:

• I wash my hands before dinner.

It does not usually mean I am washing right now. For that idea, Portuguese would normally use a different structure.

Portuguese often uses the definite article with body parts when it is already obvious whose body part is meant.

So:

• lavo as mãos = literally I wash the hands
• but the natural English meaning is I wash my hands

This is very normal in Portuguese. You do not usually need the possessive if the owner is clear from the context.

You could say as minhas mãos, but that sounds more emphatic, contrastive, or explicit than the neutral version.

Because here sabonete is being used in a general sense, like a substance or tool:

• com sabonete = with soap

If you add an article, the meaning becomes more specific:

• com um sabonete = with a soap bar / with a bar of soap
• com o sabonete = with the soap, a specific soap already known in the context

So the version without an article is the most natural if you just mean soap in general.

Yes. That is also correct and natural.

Both orders work:

• Antes de jantar, eu lavo as mãos com sabonete.
• Eu lavo as mãos antes de jantar.

The difference is mainly about focus:

• Antes de jantar, ... puts the time expression first
• ... antes de jantar gives the main action first and the time later

Portuguese is flexible here.

The comma separates the introductory time phrase from the main clause:

• Antes de jantar, eu lavo as mãos com sabonete.

With a short phrase like this, the comma is often optional. So this is also possible:

• Antes de jantar eu lavo as mãos com sabonete.

Using the comma can make the sentence feel a bit clearer or more carefully written, but both versions are acceptable.

Because Portuguese does not always need a reflexive pronoun when talking about your own body parts.

So:

• lavo as mãos = I wash my hands

is already perfectly natural.

In European Portuguese, you may also hear:

• lavo-me as mãos

That is also grammatical. If the reflexive pronoun is used in an affirmative main clause in European Portuguese, it normally comes after the verb: lavo-me.

But for a simple neutral sentence, lavo as mãos is very common and natural.

Sabonete is perfectly natural in Portugal for soap used on the body, especially a bar of toilet soap.

Sabão is a broader word and can refer more generally to soap as a cleaning substance, and in some contexts it may suggest laundry or household soap rather than hand/body soap.

So in a sentence about washing your hands, sabonete is a very good choice in European Portuguese.