Preciso da fita adesiva para fechar o envelope.

In Portuguese, the subject pronoun is often omitted when the verb ending already makes the subject clear.

• Preciso already means I need
• So Eu preciso da fita adesiva... is also correct, but eu is not necessary

This is very common in Portuguese, including European Portuguese.

Here, preciso is a verb form, not the adjective.

• preciso = I need
• It comes from the verb precisar

So in this sentence:

• Preciso da fita adesiva = I need the adhesive tape

There is also an adjective preciso meaning precise or exact, but that is a different use.

Because the verb precisar is normally used with de.

So the structure is:

• precisar de alguma coisa = to need something

That means:

• Preciso de a fita adesiva would be the full form in theory

But in Portuguese, de + a contracts to da:

• de + a = da

So:

• Preciso da fita adesiva

This is one of the most important things to notice in the sentence.

Da is a contraction of:

• de + a

In this sentence:

• de comes from the verb precisar de
• a is the feminine singular definite article the

So da fita adesiva literally has the structure of the adhesive tape, although in natural English we simply say the adhesive tape after need.

Other similar contractions are:

• de + o = do
• de + os = dos
• de + as = das

Yes, usually it suggests a specific piece or roll of tape, not just tape in general.

Compare:

• Preciso da fita adesiva = I need the adhesive tape, probably a particular one
• Preciso de fita adesiva = I need adhesive tape, in a more general sense

So the article helps show whether the noun is specific or not.

Para + infinitive is a very common way to express purpose in Portuguese.

So:

• para fechar o envelope = to close/seal the envelope
• more literally, in order to close the envelope

This structure is extremely useful:

• para abrir = to open
• para cortar = to cut
• para escrever = to write

Yes, selar could also work, but the nuance is slightly different.

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

In everyday speech, fechar o envelope is perfectly natural when you are using tape, glue, or some other method to shut it.
Selar is a bit more specifically about sealing.

So this sentence sounds natural and idiomatic.

Yes. In Portuguese, envelope is a masculine noun.

So you say:

• o envelope = the envelope
• um envelope = an envelope

That is why the sentence uses o and not a.

It is correct and understandable, but in Portugal you may also hear other everyday terms depending on the exact type of tape.

For example:

• fita adesiva = adhesive tape
• fita-cola = a very common everyday word in Portugal for sticky tape
• cola = glue, not tape

So fita adesiva is absolutely fine, but a Portuguese speaker in Portugal might often say fita-cola in casual speech.

A rough guide in European Portuguese would be something like:

• pruh-SEE-zoo duh FEE-tuh ah-dhuh-ZEE-vuh puh-ruh fuh-SHAR oo en-vuh-LOP

A few useful pronunciation notes:

• Preciso: the stress is on -ci-
• da is usually very short
• adesiva has stress on -si-
• fechar in European Portuguese sounds more like fshar than a fully open fe-char
• envelope in Portugal is often pronounced with the stress near the end

European Portuguese often reduces unstressed vowels much more than Brazilian Portuguese, so the sentence may sound more compressed than you expect.

This is the normal and natural order.

Structure:

• Preciso = main verb
• da fita adesiva = what is needed
• para fechar o envelope = purpose

So the sentence is built very naturally:

• I need + the tape + to close the envelope

You could rearrange parts in some contexts for emphasis, but the original version is the most neutral and standard.

Yes, absolutely.

Adding eu is grammatically correct. It may be used:

• for emphasis
• for contrast
• for clarity

For example:

• Eu preciso da fita adesiva, não tu. = I need the tape, not you.

But in ordinary conversation, just Preciso da fita adesiva... is very natural.