O papelão molhado pode contaminar outros recicláveis.

O is the masculine singular definite article, so here it means the.

• o papelão = the cardboard
• um papelão = a cardboard / a piece of cardboard depending on context

In this sentence, O papelão molhado refers to wet cardboard in a general, class-type sense: wet cardboard can contaminate other recyclables.

No. Papel means paper, while papelão means cardboard.

Examples:

• papel = paper
• papelão = cardboard

The ending -ão is part of the word itself here; it is not something you can freely add to papel to create a new word whenever you want. You just learn papelão as the normal word for cardboard.

In Portuguese, adjectives often come after the noun.

So:

• papelão molhado = wet cardboard
• literally: cardboard wet

This is completely normal in Portuguese. Sometimes adjectives can go before the noun, but after the noun is the basic, common position in many cases like this one.

Because molhado has to agree with papelão.

• papelão is masculine singular
• so the adjective must also be masculine singular: molhado

Agreement patterns:

• masculine singular: molhado
• feminine singular: molhada
• masculine plural: molhados
• feminine plural: molhadas

For example:

• o papelão molhado
• a caixa molhada
• os papéis molhados
• as caixas molhadas

Pode is the he/she/it/you-formal form of the verb poder, which usually means can, may, or to be able to.

Here, pode contaminar means something like:

• can contaminate
• may contaminate

In warning or instructional contexts, Portuguese often uses pode in this practical sense: something is capable of causing that result.

Examples:

• Isso pode causar problemas. = That can cause problems.
• Vidro quebrado pode machucar. = Broken glass can hurt someone.

Because after the modal verb poder, Portuguese normally uses the infinitive of the main verb.

So:

• pode contaminar = can contaminate
• pode sujar = can dirty
• pode causar = can cause

This is similar to English using can + base verb:

• can contaminate
• can damage
• can cause

Grammatically, recicláveis is originally an adjective meaning recyclable or recyclables depending on context. In this sentence, it is being used like a noun.

So:

• materiais recicláveis = recyclable materials
• recicláveis by itself = recyclables

This is very common in Portuguese. The noun is understood from context and left out.

So outros recicláveis means:

• other recyclable items
• other recyclables

Portuguese often leaves out a noun when it is obvious from context.

Here, recicláveis already works well by itself, so you do not need to say:

• outros materiais recicláveis

That longer version is possible, but the shorter version is more natural in many real-life contexts, especially on signs, labels, and recycling instructions.

So:

• outros recicláveis = natural
• outros materiais recicláveis = also correct, but more explicit

Because Portuguese often omits the article when speaking generally.

So:

• contaminar outros recicláveis = contaminate other recyclables in general

If you said os outros recicláveis, it would sound more like you are talking about a specific set of other recyclables already identified in the conversation.

Compare:

• outros recicláveis = other recyclables in general
• os outros recicláveis = the other recyclables, a specific group

A simple pronunciation guide:

• papelão ≈ pah-peh-LOWN
• recicláveis ≈ heh-see-KLAH-vays in many Brazilian accents, because word-initial r often sounds like an English h

A few important pronunciation notes:

• ão is a nasal sound. It does not sound exactly like English own; that is only a rough approximation.
• In recicláveis, the stressed syllable is clá: re-ci-CLÁ-veis.
• Final -s is pronounced like s in Brazilian Portuguese.

If you want a more IPA-style version:

• papelão: /papeˈlɐ̃w̃/
• recicláveis: /ʁesiˈklavejs/

Yes, but it means something slightly different in structure.

• O papelão molhado pode contaminar outros recicláveis.
  = Wet cardboard can contaminate other recyclables.
  Here, molhado is part of the noun phrase: the wet cardboard

• O papelão está molhado.
  = The cardboard is wet.
  Here, está molhado is the predicate: you are making a statement about the cardboard

So in your original sentence, Portuguese is packaging the idea as wet cardboard as a category or type of item.

Yes, but the original order is very natural and clear.

Original:

• O papelão molhado pode contaminar outros recicláveis.

Possible rewording:

• Outros recicláveis podem ser contaminados pelo papelão molhado.

That second version is passive:

• Other recyclables can be contaminated by wet cardboard.

Both are correct, but the original is simpler and more direct, which is why it works well for instructions or public information.