Antes de enviar a carta, usa a fita adesiva para fechar o envelope.

Because antes de is the normal pattern for before + infinitive in Portuguese.

So:

• antes de enviar = before sending / before you send
• antes de sair = before leaving
• antes de comer = before eating

English often uses either before sending or before you send, but Portuguese commonly uses antes de + infinitive.

If you use a full clause instead, the structure changes, for example with antes que + subjunctive:

• Antes que envies a carta... = Before you send the letter...

That version is possible, but antes de enviar is simpler and very common.

Yes. In European Portuguese, that is also possible.

• antes de enviar a carta = neutral, very common
• antes de enviares a carta = also correct, and it makes the tu subject more explicit

This is because Portuguese has a personal infinitive, which English does not have. In European Portuguese, learners often notice forms like:

• antes de eu enviar
• antes de enviares
• antes de ele enviar

In this sentence, enviar is the most straightforward version, but enviares would sound natural too if you are clearly speaking to tu.

Here it is a command: Use the tape...

In European Portuguese, the positive imperative for tu often looks exactly like the 3rd person singular present indicative form:

• usar → usa
• falar → fala
• abrir → abre

So usa can look like he/she uses, but in this sentence the context clearly shows it is an instruction.

A useful comparison:

• positive command: Usa a fita adesiva.
• negative command: Não uses a fita adesiva.

Notice that the negative form changes to uses.

Yes. Usa is the tu form, so this is informal singular.

More formal or more neutral options would be:

• Antes de enviar a carta, use a fita adesiva para fechar o envelope.
  This is for você or a polite singular form.

• Antes de enviar a carta, usem a fita adesiva para fechar o envelope.
  This is for vocês.

So the original sentence is the kind of instruction you might give to one person you address as tu.

Because Portuguese often omits subject pronouns when they are understood from the verb form or the context.

So instead of saying:

• Tu usa a fita adesiva...

Portuguese normally just says:

• Usa a fita adesiva...

In commands, leaving out the pronoun is especially normal. If you add tu, it usually sounds emphatic, contrastive, or unnatural in many contexts.

Because Portuguese uses definite articles more often than English.

In English, learners sometimes expect something like:

• send letter
• close envelope

But in Portuguese, with specific everyday objects, it is very natural to say:

• enviar a carta
• fechar o envelope

The articles a and o here mean the. Portuguese often sounds more complete with them.

Yes. Fita adesiva means adhesive tape or sticky tape.

It is correct and fully natural, especially in neutral or standard language. In Portugal, people also often say:

• fita-cola

So both are good, but they feel slightly different:

• fita adesiva = more neutral / descriptive
• fita-cola = very common in everyday speech

Para + infinitive expresses purpose:

• usa a fita adesiva para fechar o envelope
  = use the tape to close the envelope

This is a very common structure in Portuguese.

As for the verb:

• fechar o envelope = close the envelope
• selar o envelope = seal the envelope

Both can work, but fechar is very natural in a practical instruction like this, especially when you are physically closing it with tape. Selar sounds a bit more specifically like sealing it.

Yes, often you could.

• enviar a carta = more standard and direct
• mandar a carta = also common in many contexts

But enviar is a very good choice here because it fits clear instructional language. It also works well across many contexts, including post, parcels, and even digital things like emails.

So mandar is possible, but enviar is a strong standard option.

Yes to both.

The comma appears because Antes de enviar a carta is an introductory time clause. Portuguese usually separates that kind of opening element with a comma:

• Antes de enviar a carta, usa a fita adesiva...

You can also put that part at the end:

• Usa a fita adesiva para fechar o envelope antes de enviar a carta.

That version is also correct. The meaning stays basically the same, but the emphasis changes slightly:

• opening with Antes de enviar a carta highlights the time sequence first
• putting it at the end makes the instruction itself come first