Ik zet de wekker af zodra hij afgaat.

Because the sentence contains two different separable verbs:

• afzetten = to switch off / turn off
• afgaan = to go off / start ringing

So:

• Ik zet de wekker af = I turn the alarm off
• zodra hij afgaat = as soon as it goes off

Even though both verbs contain af, they are different verbs with different meanings.

Because afzetten is a separable verb.

In a normal main clause, Dutch separates the verb:

• Ik zet de wekker af.
• not: Ik afzet de wekker.

The conjugated part (zet) goes in the usual verb position, and the prefix (af) moves to the end of the clause.

This happens with many Dutch separable verbs:

• opstaan → Ik sta op
• aandoen → Ik doe het licht aan
• uitzetten → Ik zet de tv uit

Because in a subordinate clause, a separable verb usually becomes one whole word again.

Compare:

• Main clause: De wekker gaat af.
• Subordinate clause: ... zodra de wekker afgaat.

So:

• gaat af in a main clause
• afgaat in a subordinate clause

That is a very common Dutch pattern.

Because zodra introduces a subordinate clause.

In Dutch subordinate clauses, the finite verb normally goes to the end:

• Ik kom als hij belt.
• Ik blijf thuis omdat ik moe ben.
• Ik zet de wekker af zodra hij afgaat.

So after zodra, you expect the verb to come at the end of that clause.

Zodra means as soon as.

It introduces something that happens immediately after something else:

• Ik bel je zodra ik thuis ben. = I’ll call you as soon as I’m home.
• Ik zet de wekker af zodra hij afgaat. = I switch off the alarm as soon as it goes off.

It is a little more specific and immediate than als (when / if).

Because wekker is a de-word: de wekker.

In traditional Dutch grammar, de-words can be referred to with hij (or sometimes zij for clearly feminine nouns), while het-words can be referred to with het.

So:

• de wekker → hij
• het alarm → often het

That said, in everyday Dutch, people often avoid strict grammatical pronouns for objects and may repeat the noun instead:

• Ik zet de wekker af zodra de wekker afgaat.

But hij is completely normal and correct here.

Because Dutch usually needs an article before a singular countable noun.

So you normally say:

• de wekker = the alarm clock / the alarm

You cannot usually say just wekker by itself in a sentence like this.

Here de wekker can refer to the specific alarm that is understood from context, even if English might sometimes say just the alarm.

Because afzetten is the fixed verb used here: to switch off / turn off.

Even though the basic verb zetten often means to put or to place, in combinations like this the full separable verb has its own meaning:

• afzetten = switch off
• aanzetten = switch on
• uitzetten = turn off / switch off

So you should learn afzetten as one vocabulary item, not interpret it word by word.

They describe opposite sides of the alarm event:

• de wekker gaat af = the alarm goes off / starts ringing
• ik zet de wekker af = I turn the alarm off / stop it

So the sentence means that the speaker turns the alarm off immediately when it starts ringing.

This pairing is very common in Dutch.

Yes, in many contexts uitzetten can also mean turn off.

However, with alarms and alarm clocks, afzetten is especially common for stopping the alarm once it starts ringing.

So:

• de wekker afzetten = stop the alarm
• een apparaat uitzetten = switch off a device

Both may be understood, but afzetten fits this sentence very naturally.

Because Dutch often uses the present tense for:

• habits and routines
• general truths
• near-future situations

This sentence sounds like a routine or normal action:

• Ik zet de wekker af zodra hij afgaat.

That can mean something like Whenever it goes off, I turn it off or I turn it off as soon as it goes off.

So the present tense is completely natural here.

It can mean either, depending on context.

In everyday Dutch, de wekker often refers to:

• the alarm clock as an object, or
• the alarm setting/alarm sound associated with it

So in this sentence, it is best understood as the alarm that goes off and that the speaker turns off.

Dutch often uses the same word for the device and the alarm function.