As botas de caminhada estão molhadas.

As is the feminine plural definite article, meaning the.

It matches botas, because:

• bota = boot
• botas = boots
• bota is feminine
• botas is plural

So:

• a bota = the boot
• as botas = the boots

Portuguese uses articles more often than English, so as sounds completely natural here.

In Portuguese, every noun has a grammatical gender, usually masculine or feminine.

Bota is simply a feminine noun. That is something you learn together with the word:

• a bota = the boot

When it becomes plural, the article and other words must agree with it:

• as botas

This is grammatical gender, not biological gender.

De caminhada describes the type of boots.

Literally, it is something like of walking / for walking, but in natural English the whole phrase means hiking boots or walking boots.

So:

• botas = boots
• botas de caminhada = hiking boots / walking boots

This is a very common Portuguese structure:

• sapatos de corrida = running shoes
• roupa de inverno = winter clothes
• botas de caminhada = hiking boots

Because Portuguese often uses a noun after de to classify or describe something.

Here:

• caminhar = to walk / to hike
• caminhada = walk / hike / hiking

So botas de caminhada is a noun-based expression, like a category label.

You could also hear botas para caminhar, which means something like boots for walking, but botas de caminhada sounds more like the normal name of the item.

Because after de, Portuguese often uses a singular noun to indicate the type or purpose of something.

So de caminhada means for hiking / hiking-type as a category, not of hikes in a countable sense.

Compare:

• botas de caminhada = hiking boots
• sapatos de ténis = tennis shoes
• óculos de sol = sunglasses

The second noun does not have to match the number of the first noun.

Because estar is used for a state or condition, while ser is used for something more essential, defining, or permanent.

Here, the boots are wet right now. That is a condition, so Portuguese uses estar:

• estão molhadas = are wet

If you said são molhadas, it would sound unnatural in this context, because being wet is not what the boots are by nature; it is just their current state.

This is one of the most important differences in Portuguese:

• ser = to be in an essential sense
• estar = to be in a temporary or current-state sense

Because the subject is plural:

• as botas = the boots

Portuguese verbs change depending on the subject. The verb estar in the present tense goes like this:

• eu estou = I am
• tu estás = you are
• ele/ela está = he/she is
• nós estamos = we are
• vocês/eles/elas estão = you/they are

Since botas is plural, the sentence needs estão.

Because adjectives must agree with the noun they describe.

The noun is botas, which is:

• feminine
• plural

So the adjective must also be feminine plural:

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

That is why we say:

• As botas ... estão molhadas.

Yes. It is still an adjective.

In estão molhadas, the adjective describes the subject as botas through the verb estar. This is called a predicate adjective in English grammar terms.

So:

• as botas = subject
• estão = verb
• molhadas = adjective describing the subject

Even though it comes after the verb, it still has to agree with botas.

Usually, in normal Portuguese, no. The sentence sounds more natural with the article:

• As botas de caminhada estão molhadas.

Leaving out the article may sound incomplete, headline-like, or unnatural in ordinary speech.

Portuguese often uses definite articles where English may not stress them as much.

Yes, if the context is already clear.

Portuguese often leaves out the subject when it is understood from the situation. So if everyone already knows you are talking about the hiking boots, you can say:

• Estão molhadas. = They’re wet.

The plural verb estão already tells you the subject is plural, and molhadas tells you it is feminine plural.

The word order is very straightforward:

• As botas de caminhada = subject
• estão = verb
• molhadas = adjective/complement

So the pattern is:

Subject + verb + adjective

This is a very common structure in Portuguese.

In European Portuguese, estão is pronounced roughly like sh-TOWN with a nasal ending, though that English approximation is not exact.

A few important points:

• the first e is often very weak
• the s before t sounds like sh
• ão is nasal, which is very important in Portuguese

So it does not sound like a clear English es-tow.

If you want a rough learner-friendly approximation, think:

• estão ≈ sh-TOWN
  but with a Portuguese nasal sound at the end

If you were talking about one boot, everything would change to singular feminine:

• A bota de caminhada está molhada.

Compare:

• As botas de caminhada estão molhadas. = plural
• A bota de caminhada está molhada. = singular

Notice that article, verb, and adjective all change to match the noun.