Separe o vidro, o plástico e o metal.

Separe is a command form of the verb separar.

• separar = the infinitive, to separate
• separe = an imperative form, used to tell someone separate...

In Brazilian Portuguese, the affirmative imperative for você uses the same form as the present subjunctive:

• que você separe
• Separe!

So in this sentence, separe means the speaker is giving an instruction.

By contrast:

• separar is not a command; it is just the dictionary form
• separa is the command form usually associated with tu in standard grammar

It is understood, not stated.

Portuguese often leaves the subject out when it is clear from the verb form or the situation. Commands work a lot like English here:

• English: Separate the glass...
• Portuguese: Separe o vidro...

In both languages, the person being addressed is understood as you.

If you wanted to make it explicit, you could say:

• Você, separe o vidro, o plástico e o metal.

But that is usually unnecessary.

In Brazilian Portuguese, separe is often a neutral everyday command because it matches você, which is the normal singular you in much of Brazil.

So this sentence is not automatically very formal. It can sound:

• neutral
• practical
• common on signs, labels, and instructions

The same form can also be used with o senhor or a senhora, which are more formal:

• O senhor, separe o vidro...

So the form itself can fit different levels of formality, depending on context.

Because all three nouns are masculine singular, so they take the definite article o.

• o vidro
• o plástico
• o metal

Portuguese uses articles more often than English does, especially with general categories and materials. English often says:

• glass, plastic, and metal

But Portuguese naturally says:

• o vidro, o plástico e o metal

Repeating the article before each item also makes the list sound complete and natural.

Because they refer to materials/categories in general, not to individual countable objects.

So:

• o vidro = glass as a material/category
• o plástico = plastic as a material/category
• o metal = metal as a material/category

This is very common in Portuguese, especially in recycling or sorting instructions.

If you used plurals, it would sound more like you were talking about specific groups of items:

• os vidros, os plásticos e os metais

That is possible in some contexts, but for a general recycling instruction, the singular is more natural.

Yes. Plástico can be:

• an adjective: um copo plástico = a plastic cup
• a noun: o plástico = plastic

In this sentence, it is clearly a noun, because it names one of the materials being separated.

The same thing happens in English too:

• plastic bag → adjective
• Plastic takes a long time to decompose → noun

Yes, you could, especially in:

• short signs
• labels
• very concise instructions

But it sounds more telegraphic or headline-like.

Compare:

• Separe o vidro, o plástico e o metal. → fuller, more natural as a complete sentence
• Separe vidro, plástico e metal. → shorter, more compressed

Both are understandable, but the version with the articles is very natural in standard Brazilian Portuguese.

That is the normal way to write a simple list in Portuguese.

• o vidro, o plástico e o metal

Just like in standard Portuguese punctuation, you usually do not put a comma before e in a basic list.

So this works like:

• item 1, item 2 and item 3

A comma before e can appear in special cases, but not usually in a simple list like this.

A simple stress guide is:

• se-PA-re
• VI-dro
• PLÁS-ti-co
• me-TAL

A rough Brazilian Portuguese pronunciation guide would be:

• Separe ≈ seh-PAH-ree
• o vidro ≈ oo VEE-droo
• o plástico ≈ oo PLAS-chee-coo or oo PLAS-tee-coo depending on accent
• o metal ≈ oo meh-TAU

A few useful pronunciation notes:

• The final unstressed e in separe often sounds like ee in Brazil.
• The final unstressed o in vidro often sounds like oo.
• The accent mark in plástico shows that the stress goes on plás.
• In many Brazilian accents, final l in metal sounds like w.

Because Portuguese instructions on signs often address each reader individually.

So even in a public place, a singular command like:

• Separe o vidro, o plástico e o metal.

can be completely natural.

If you were clearly speaking to a group, you could also use the plural command:

• Separem o vidro, o plástico e o metal.

But the singular is very common in general instructions, posters, and notices.