Tegen morgen zal die vlek vast verdwenen zijn, want ik heb sterk wasmiddel gebruikt.

Tegen morgen means something like by tomorrow.

It expresses a deadline or point in time by which something will be true. So here, the idea is that by the time tomorrow arrives, the stain will probably be gone.

A few useful comparisons:

• morgen = tomorrow
• tegen morgen = by tomorrow / for tomorrow
• tot morgen = until tomorrow

So tegen morgen is not just naming a time; it sets an expected result before that time.

This is the future perfect in Dutch.

Structure:

• zullen
  • past participle + infinitive of hebben or zijn

Here:

• zal = future auxiliary, will
• verdwenen = past participle of verdwijnen
• zijn = auxiliary used with this verb

So zal ... verdwenen zijn means will have disappeared / will be gone.

Dutch often uses this structure when talking about something that will be completed before a future moment:

• Morgen zal hij vertrokken zijn. = By tomorrow he will have left.
• Volgende week zal het klaar zijn. = It will be finished by next week.

Because verdwijnen is one of the verbs that forms its perfect tenses with zijn, not hebben.

Many Dutch intransitive verbs involving:

• movement
• change of state
• appearing/disappearing

often take zijn.

Examples:

• Hij is gekomen. = He came / has come.
• Ze is vertrokken. = She left / has left.
• De vlek is verdwenen. = The stain disappeared / has disappeared.

So:

• present perfect: De vlek is verdwenen
• future perfect: De vlek zal verdwenen zijn

Yes, vast can mean fixed, firm, or stuck, but here it means something different.

In this sentence, vast is an adverb meaning something like:

• probably
• surely
• I expect
• no doubt

So die vlek zal vast verdwenen zijn means something like:

• that stain will probably be gone
• that stain is sure to have disappeared

This use of vast is very common in spoken and everyday Dutch.

Examples:

• Hij komt vast later. = He’ll probably come later.
• Dat weet ze vast al. = She probably already knows that.

Dutch main clauses follow the verb-second rule.

That means the finite verb must come in the second position. If you begin the sentence with something other than the subject, the subject comes after the finite verb.

Here the first element is:

• Tegen morgen

So the finite verb must come next:

• zal

Then the subject:

• die vlek

Then the rest:

• vast verdwenen zijn

So the pattern is:

• Tegen morgen | zal | die vlek | vast verdwenen zijn

If you started with the subject instead, you could say:

• Die vlek zal tegen morgen vast verdwenen zijn.

That is also grammatical.

Both are possible, but they are not exactly the same.

• die = that
• deze = this

So die vlek points to a stain that is a bit more distant, already known, or being referred to from context: that stain.

Dutch demonstratives work like this:

For de-words:

• deze = this
• die = that

For het-words:

• dit = this
• dat = that

Since vlek is a de-word (de vlek), the forms are deze and die.

This is about adjective endings in Dutch.

Wasmiddel is a het-word: het wasmiddel.

After an indefinite article omitted or in cases like this where the noun is used without an article, a singular het-word often takes the adjective without -e:

• sterk wasmiddel

Compare:

• het sterke wasmiddel = the strong detergent
• een sterk wasmiddel = a strong detergent
• sterk wasmiddel = strong detergent

So the missing -e is because this is a singular neuter noun used indefinitely.

Because this clause is in the perfect tense.

In a Dutch main clause with a perfect tense:

• the finite auxiliary goes in second position
• the past participle usually goes to the end

So:

• ik = subject
• heb = finite auxiliary
• sterk wasmiddel = object
• gebruikt = past participle at the end

Pattern:

• Ik heb koffie gedronken.
• Zij heeft een boek gekocht.
• Ik heb sterk wasmiddel gebruikt.

This is very normal Dutch word order.

Both can mean because, but they behave differently.

want is a coordinating conjunction. It introduces a new main clause, so normal main-clause word order stays the same:

• ..., want ik heb sterk wasmiddel gebruikt.

Here the finite verb is still in second position:

• ik heb ... gebruikt

omdat is a subordinating conjunction. It sends the verbs to the end of the clause:

• ..., omdat ik sterk wasmiddel heb gebruikt.

So both are possible, but the word order changes.

A rough difference in feel:

• want often sounds a bit more like giving an explanation afterward
• omdat can sound a bit tighter or more explicitly subordinate

Formally, verdwenen is the past participle of verdwijnen.

In this sentence, it is part of the verbal construction verdwenen zijn. So it is mainly functioning as part of the verb phrase, not just as an ordinary adjective.

You can think of:

• De vlek is verdwenen. = perfect/resultative idea: The stain has disappeared / is gone
• De verdwenen vlek would be odd in most contexts, because verdwenen is not commonly used as a normal attributive adjective there

So here it is best understood as part of the tense construction, especially in zal verdwenen zijn.

Yes, you could, and Dutch often does this.

Dutch frequently uses the present or present perfect to talk about the future when the context already makes the time clear.

So these are both possible:

• Tegen morgen zal die vlek vast verdwenen zijn.
• Tegen morgen is die vlek vast verdwenen.

The version with zal ... zijn sounds a bit more explicitly predictive or formal: will have disappeared.

The version with is verdwenen can sound a bit more natural and idiomatic in everyday Dutch, especially when tegen morgen already clearly places it in the future.

It is not absolute certainty.

Two things affect the nuance:

• zal ... zijn can express a future prediction
• vast adds the idea of probably / surely / I’m pretty sure

So the speaker sounds confident, but not mathematically certain. It is more like:

• That stain will probably be gone by tomorrow, because I used strong detergent.

If the speaker wanted to sound more definite, Dutch might use wording without vast, depending on context.

Yes. Dutch allows some flexibility, especially with time expressions and adverbs.

For example, these are possible:

• Tegen morgen zal die vlek vast verdwenen zijn.
• Die vlek zal tegen morgen vast verdwenen zijn.
• Die vlek zal vast tegen morgen verdwenen zijn.

The basic meaning stays similar, but the emphasis changes slightly.

• starting with Tegen morgen emphasizes the deadline
• starting with Die vlek emphasizes the stain itself

Dutch learners often find this flexibility tricky, but the key rule is still that in a main clause the finite verb stays in second position.