Er zit een vlek van tomaat op mijn blouse, dus ik gebruik extra wasmiddel.

Here er is not the same as there meaning a place. It is an existential er, used when Dutch introduces the existence or presence of something.

So:

• Er zit een vlek op mijn blouse. = There is a stain on my blouse.

In neutral Dutch, this er is very common. Without it, the sentence would sound incomplete or marked in this context.

Dutch often uses verbs like zitten, staan, and liggen to describe where something is.

In this sentence, zitten is used because the stain is thought of as being on the blouse, almost stuck to it.

So:

• Er zit een vlek op mijn blouse. = natural, idiomatic Dutch
• Er is een vlek op mijn blouse. = possible, but less vivid and less idiomatic here

A useful idea is:

• zijn = to exist / to be
• zitten = to be located somewhere, often attached or on a surface

Yes. Een vlek van tomaat means a stain from tomato or a tomato stain.

The van here means from / caused by, not possession.

You can also say:

• een tomatenvlek = a tomato stain

Both are understandable. The version with van is very transparent for learners, while the compound tomatenvlek is also very natural Dutch.

If you want to be more specific, you might also hear:

• een vlek van tomatensaus
• een tomatensausvlek

After van, Dutch often uses a singular noun to indicate the substance or source of something.

So:

• een vlek van tomaat = a stain caused by tomato

This is similar to saying made of wood or full of water in English, where English also does not always use a plural.

But in a compound noun, Dutch often uses a linking form:

• tomatenvlek

So both patterns are normal, just built differently.

Because op means on, and a stain is physically on the surface of the blouse.

Compare:

• op mijn blouse = on my blouse
• in mijn blouse = in my blouse / inside my blouse
• aan mijn blouse = attached to my blouse

So for stains, op is the normal choice.

Because dus here works like a coordinating connector meaning so.

It links two main clauses:

• Er zit een vlek ... , dus ik gebruik extra wasmiddel.

After this dus, Dutch keeps normal main-clause order:

• ik gebruik

Compare that with daarom:

• Er zit een vlek op mijn blouse. Daarom gebruik ik extra wasmiddel.

After daarom, inversion happens because daarom takes the first position in the clause:

• Daarom gebruik ik ...

So a simple rule is:

• dus + subject + verb
• daarom + verb + subject (if daarom starts the clause)

Because extra normally stays extra in Dutch. It does not take the usual adjective ending here.

So you say:

• extra wasmiddel
• extra tijd
• extra hulp

Not:

• extrae wasmiddel

This is just how extra behaves in modern Dutch.

Here wasmiddel is being used as a mass noun, like detergent in English.

So:

• ik gebruik extra wasmiddel = I use extra detergent

You are talking about more of the substance, not one individual item.

If you said een wasmiddel, that would usually mean:

• a detergent
• a type/brand of detergent
• one detergent product

But in this sentence, the mass-noun use is the natural choice.

Yes. The Dutch present tense is used quite broadly. It can mean:

• a general habit
• something happening now
• a near-future action

So ik gebruik extra wasmiddel can mean:

• I use extra detergent
• I’m using extra detergent
• I’ll use extra detergent

The exact meaning depends on context. In this sentence, it most naturally means something like so I use / am using extra detergent.

Yes, if the clothing item is different.

For example:

• op mijn shirt
• op mijn hemd
• op mijn T-shirt

But blouse specifically means blouse, so it is the right word if that is the garment you mean. The grammar of the sentence stays the same.