Ik wil van jas wisselen, want deze is nat.

Because willen is a modal verb.

In a normal Dutch main clause, the finite verb goes in second position. Here that verb is wil. The other verb, wisselen, stays in the infinitive and goes to the end:

• Ik wil van jas wisselen.
• Zij kan morgen komen.
• Wij moeten nu gaan.

So the pattern is:

subject + finite verb + rest + infinitive

Here van is part of the expression van ... wisselen, which means to switch from one ... to another.

So van jas wisselen does not mean changing the jacket itself; it means changing which jacket you are wearing.

It suggests there is another jacket available. In other words, the speaker wants to stop wearing this jacket and wear a different one.

Dutch often leaves out the article in certain set expressions, especially when talking about changing clothes or switching from one item of a type to another.

So van jas wisselen is understood more like:

• switch jackets
• change jacket

The focus is on the type of item, not on identifying one specific jacket with de, een, or mijn.

A learner might expect something like van een jas wisselen, but that is not how this expression is usually built.

Because jas is a de-word.

In Dutch, the demonstratives match the noun:

• deze for de-words
• dit for het-words

Since de jas is correct, the matching demonstrative is deze.

So:

• de jas → deze
• het huis → dit

Here deze means this one or this jacket.

Yes. That is very common in Dutch.

Instead of saying deze jas, Dutch can simply say deze when the noun is already clear from context.

So:

• deze is nat = this one is wet
• understood as this jacket is wet

English does the same thing with this one.

Because want is a coordinating conjunction, not a subordinating one.

That means the clause after want keeps normal main-clause word order:

• want deze is nat

Compare that with omdat, which does cause subordinate word order:

• ..., omdat deze nat is

So:

• want → normal order
• omdat → verb goes to the end

Both can express because, but they behave differently grammatically.

• want connects two main clauses:
  • Ik wil van jas wisselen, want deze is nat.
• omdat introduces a subordinate clause:
  • Ik wil van jas wisselen, omdat deze nat is.

In everyday meaning, both work here. The biggest difference for learners is the word order.

Yes. Nat is the normal everyday word for wet.

It can describe something soaked, rainy, splashed, or generally not dry.

Examples:

• Mijn schoenen zijn nat.
• Zijn jas is nat van de regen.

If something is only slightly damp, Dutch can also use vochtig, but nat is the usual basic word.

Yes, it is understandable and natural if the speaker means I want to switch to another jacket because this one is wet.

That said, many speakers might also say:

• Ik wil een andere jas aantrekken, want deze is nat.

That version focuses more directly on putting on another jacket.
The original sentence focuses on switching jackets. Both are fine; the nuance is just slightly different.