Het alarm gaat af als de deur open blijft.

Dutch nouns take either de or het as their definite article.

• alarm is a het-word, so you say het alarm
• deur is a de-word, so you say de deur

Unfortunately, this usually has to be learned word by word. For English speakers, it can feel a bit like grammatical gender, although het and de do not work exactly like the in English.

Gaat af comes from the verb afgaan, which is a separable verb.

The full verb is:

• afgaan = to go off

In a normal main clause, the verb splits:

• Het alarm gaat af.

Here:

• gaat = the conjugated part
• af = the separable prefix

This is very common in Dutch. Compare:

• Ik sta op from opstaan
• Hij belt terug from terugbellen

So in this sentence, gaat af functions together as one verb meaning goes off.

In this sentence, als means if or when, depending on context.

Here it introduces a condition:

• als de deur open blijft = if the door stays open

Dutch often uses als for real or likely conditions in everyday language.

A few useful comparisons:

• als = if / when
• wanneer = when
• indien = if, but more formal

So als is the most natural everyday choice here.

Because als de deur open blijft is a subordinate clause.

In Dutch subordinate clauses, the conjugated verb usually moves toward the end. That is why you get:

• de deur open blijft

instead of the main-clause order:

• de deur blijft open

So the pattern is:

• main clause: De deur blijft open.
• subordinate clause: ... als de deur open blijft.

This is one of the most important word-order patterns in Dutch.

Open is an adjective describing the state of the door. With verbs like blijven, Dutch often places that descriptive word before the final verb in a subordinate clause.

So:

• De deur blijft open. = main clause
• ... als de deur open blijft. = subordinate clause

You can think of open blijven as a unit meaning to remain open or to stay open.

Dutch normally uses open for the state of a door, window, shop, etc.

• de deur is open = the door is open
• de deur blijft open = the door stays open

Geopend exists, but it is less natural here. It often sounds more formal or more like opened in the sense of having been opened, rather than simply being in the open state.

So for ordinary everyday Dutch, open blijft is the normal choice.

Blijft is the third person singular present tense of blijven.

Conjugation:

• ik blijf
• jij blijft
• hij/zij/het blijft

Since de deur is singular, Dutch uses blijft.

Yes, that is grammatical.

• Als de deur open blijft, gaat het alarm af.
• Wanneer de deur open blijft, gaat het alarm af.

Both are possible, but als often sounds more natural for a condition like this. Wanneer can sound a bit more like when(ever) or slightly more formal, depending on context.

So als is usually the best everyday option here.

Then the main clause has inversion:

• Als de deur open blijft, gaat het alarm af.

Notice that it becomes gaat het alarm af, not het alarm gaat af.

That is because when a Dutch sentence begins with something other than the subject, the finite verb comes before the subject in the main clause.

Compare:

• Het alarm gaat af als de deur open blijft.
• Als de deur open blijft, gaat het alarm af.

Both mean the same thing.

Dutch often uses the present tense for general rules, automatic results, and things that happen whenever a condition is met.

So this sentence describes a general fact:

• if the door stays open, the alarm goes off

English does the same thing:

• If the door stays open, the alarm goes off.

So the present tense here is completely normal.

No. Afgaan has several meanings depending on context.

Common ones include:

• Het alarm gaat af. = the alarm goes off
• De bom ging af. = the bomb exploded
• Op iets afgaan = to rely on something / go by something

So in this sentence, the meaning is specifically the alarm-related one: to go off.

Yes, it sounds natural and idiomatic.

A Dutch speaker would easily understand it, and it is a good example of:

• a separable verb: afgaan
• a subordinate clause with als
• normal Dutch verb-final word order in that clause: open blijft

So it is a very useful model sentence for learners.