Wij isoleren de spouwmuur.

Yes. Wij and we both mean we.

• we is more common in everyday speech.
• wij is the fuller, stressed form.
• wij is often used for emphasis, contrast, or in more careful speech/writing.

So all of these are possible:

• Wij isoleren de spouwmuur.
• We isoleren de spouwmuur.

They both mean the same thing here. A learner will probably hear we more often in conversation.

Yes. Isoleren is both:

• the infinitive: to insulate
• the present tense plural form: we/you/they insulate

That is normal in Dutch. For regular verbs, the plural present tense is the same as the infinitive.

For isoleren:

• ik isoleer = I insulate
• jij/je isoleert = you insulate
• hij/zij/het isoleert = he/she/it insulates
• wij/we isoleren = we insulate
• jullie isoleren = you all insulate
• zij/ze isoleren = they insulate

So in this sentence, isoleren is not an infinitive. It is the present tense verb agreeing with wij.

Spouwmuur is a compound noun meaning cavity wall.

It is made of:

• spouw = cavity / hollow space
• muur = wall

Dutch very often joins nouns together into one word, where English might use two words or a phrase. So:

• spouwmuur = cavity wall
• not spouw muur

This is very normal Dutch spelling.

Because spouwmuur is a de-word.

In Dutch, singular nouns usually take either:

• de
• or het

There is not always a logical reason you can see from the meaning alone; you often just have to learn the article with the noun.

So you should learn it as:

• de spouwmuur

In the plural, it would be:

• de spouwmuren

since all plural nouns take de.

Here it is basically like English:

• Wij = subject
• isoleren = verb
• de spouwmuur = object

So the structure is:

• Subject + Verb + Object

However, Dutch main clauses follow the verb-second rule. That means the finite verb usually comes in the second position.

Because the subject is first here, the sentence looks simple:

• Wij isoleren de spouwmuur.

But if another element comes first, the verb still stays second:

• Morgen isoleren wij de spouwmuur. = Tomorrow we are insulating the cavity wall.

So this sentence is simple, but it still follows a very important Dutch rule.

Yes. The Dutch present tense often covers both meanings.

So Wij isoleren de spouwmuur can mean:

• We insulate the cavity wall
• We are insulating the cavity wall

Dutch does have ways to emphasize an ongoing action, but the simple present is very common and often enough.

So you should not assume Dutch always needs a separate form like English are insulating.

A rough guide is:

• spouw: the ou sounds roughly like the vowel in cow
• muur: the uu is not a normal English sound; it is a long front rounded vowel

A rough English-friendly approximation would be something like:

• spow-muur

But that is only approximate.

A few notes:

• spouw sounds like sp
  • a vowel similar to ow in cow
• muur is not the same as English moor
• the r may sound lighter or less strongly pronounced than in many English accents

If you want to improve pronunciation, the hardest part is usually uu in muur.

Because Dutch writes compound nouns as a single word.

This is one of the most noticeable differences from English. English often uses:

• two words
• or a hyphen

Dutch usually combines them:

• spouwmuur
• huisdeur = house door / front door
• keukentafel = kitchen table

So if two nouns form one idea, Dutch usually writes them together.

No. Wij is a subject pronoun, so it means we.

If you want us, Dutch uses ons.

Compare:

• Wij isoleren de spouwmuur. = We insulate the cavity wall.
• Hij helpt ons. = He helps us.

So in this sentence, wij can only be we.

Yes. It is a normal Dutch verb, even though it looks familiar to English speakers.

Dutch has many words of Latin or French origin that resemble English words, especially in technical or formal contexts. Isoleren is one of them.

That can be helpful, but you still need to learn the Dutch grammar around it, such as:

• the Dutch pronunciation
• the Dutch verb endings
• Dutch word order

So the word may look easy, but it still behaves like a Dutch verb.

Yes, the sentence is natural.

A Dutch speaker might very often say:

• We isoleren de spouwmuur.

rather than Wij isoleren de spouwmuur, because we is more common in ordinary speech.

But your sentence is still perfectly correct and natural, especially if:

• the speaker is being clear or careful
• there is emphasis on we
• it appears in writing or instruction-like language

So the sentence is good Dutch. The most casual spoken version would simply often use we.