Construímos o abrigo de montanha.

Construímos is the 1st person plural form of the verb construir (to build).

So it matches the subject we, even though nós is not written.

• eu construo = I build
• tu constróis = you build
• ele/ela constrói = he/she builds
• nós construímos = we build / we built

In this sentence, construímos is the finite verb and o abrigo de montanha is its direct object.

It can mean either one.

With construir, the nós form is spelled the same in the:

• present indicative: we build
• preterite: we built

So Construímos o abrigo de montanha can mean:

• We build the mountain shelter
  or
• We built the mountain shelter

Usually, context tells you which one is meant. For example:

• Todos os verões, construímos abrigos. = Every summer, we build shelters.
• No ano passado, construímos o abrigo. = Last year, we built the shelter.

Because Portuguese often omits subject pronouns when the verb ending already makes the subject clear.

The ending -mos shows that the subject is we. So:

• Construímos o abrigo de montanha.
• Nós construímos o abrigo de montanha.

Both are correct.

Adding nós usually gives:

• emphasis
• contrast
• extra clarity in context

For example:

• Nós construímos o abrigo, mas eles financiaram o projeto.
  = We built the shelter, but they financed the project.

Because o is the definite article, meaning the.

Portuguese uses articles very naturally and often more often than English. Here, o abrigo suggests a specific shelter that the speaker and listener can identify.

• o abrigo = the shelter
• um abrigo = a shelter

So:

• Construímos o abrigo de montanha. = We built the mountain shelter.
• Construímos um abrigo de montanha. = We built a mountain shelter.

Because de montanha here works like an adjectival expression meaning something like mountain or for the mountains.

So abrigo de montanha means:

• mountain shelter
• mountain hut
• shelter in the mountains

But da montanha means of the mountain, referring to a specific mountain.

Compare:

• abrigo de montanha = mountain shelter / shelter for mountain use
• abrigo da montanha = the shelter of the mountain

So in your sentence, de montanha describes the type of shelter, not possession.

Because in expressions like abrigo de montanha, the noun after de often functions almost like an adjective.

So de montanha describes the kind of shelter, just like English mountain in mountain shelter.

This is very common in Portuguese:

• sapatos de corrida = running shoes
• óculos de sol = sunglasses
• casa de campo = country house
• abrigo de montanha = mountain shelter

Even if the idea is connected with mountains in general, the singular is often the normal form in this type of expression.

A helpful approximation is:

constru-EE-mush

The main stress falls on í:

• cons-tru-Í-mos

A few useful points for European Portuguese:

• the í is clearly stressed
• the final -s in -mos often sounds like sh at the end of an utterance
• the vowels may be more reduced than an English speaker expects

If you like a rougher Portugal-style approximation:

• consh-troo-EE-moosh

But the most important thing is to stress í: construÍmos.

A simple approximation is:

uh-BREE-goo

More specifically for European Portuguese:

• the a is usually reduced, more like uh
• the stress is on bri
• the g is a hard g, as in go

So:

• a-BRI-go

The word does not have the English j sound.

It is a very natural expression, yes.

It means a shelter or hut in the mountains, especially one used for protection, rest, or refuge.

Depending on context, you may also hear:

• refúgio de montanha = mountain refuge / mountain hut
• cabana de montanha = mountain cabin
• abrigo by itself = shelter

The exact best word depends on the kind of place:

• abrigo often emphasizes protection or shelter
• refúgio can sound more like a refuge/hut used by hikers or climbers
• cabana sounds more like a cabin

Yes, but the sentence you have is the most neutral and standard order:

• Construímos o abrigo de montanha.

That is:

• verb + object

You could also say:

• Nós construímos o abrigo de montanha.

This adds the subject explicitly, often for emphasis.

Other word orders are possible in special contexts, but they are less neutral and usually depend on style, emphasis, or discourse context. For a learner, the basic pattern to remember is:

• [verb] + [article + noun phrase]

No. For were building, you need the imperfect of estar plus the gerund:

• Estávamos a construir o abrigo de montanha.

In European Portuguese, the usual progressive form is:

• estar a + infinitive

So:

• Construímos = we build / we built
• Estávamos a construir = we were building
• Estamos a construir = we are building

That distinction is very important in Portuguese.