Learn Hardcore Dutch: Wie ziek is, die kan zich via de app tijdig afmelden. - Whoever is ill can deregister in time via the app.

Wie … die … is a correlative pattern meaning “whoever … that person …”. The second die resumes or emphasizes the subject introduced by wie.
You can drop die: Wie ziek is, kan zich via de app tijdig afmelden. That’s fully correct and a bit leaner.
Keeping die is a touch more formal/emphatic and common in written instructions or announcements. It doesn’t change the meaning.

Why are the verbs singular (is, kan) after wie? Could it be plural?

With wie used as “whoever,” Dutch takes third-person singular agreement: wie ziek is, (die) kan ….
If you want a clearly plural subject, rephrase: Zij die ziek zijn, (die) kunnen zich via de app tijdig afmelden.

Can I say the more conversational version with als je?

Yes: Als je ziek bent, kun je je via de app tijdig afmelden.
That’s the everyday, direct way to say it. The original wie … (die) … sounds slightly more formal/institutional.

Why is afmelden at the end? Isn’t it a separable verb?

Afmelden is separable, so in a simple present without a modal you split it: Hij meldt zich via de app tijdig af.
With a modal like kan, you use the infinitive at the end and keep the prefix attached: Hij kan zich via de app tijdig afmelden.
In the perfect: Hij heeft zich via de app tijdig afgemeld.

Why is the reflexive pronoun zich there? How does it change with person?

Zich afmelden means “to sign off/deregister oneself.” It’s reflexive when the subject is the person cancelling their own attendance.
Person forms with a modal:
- ik: ik kan me afmelden
- jij/je: jij kunt je afmelden
- hij/zij: hij/zij kan zich afmelden
- wij: wij kunnen ons afmelden
- jullie: jullie kunnen je afmelden
- zij (pl): zij kunnen zich afmelden
- u: u kunt zich afmelden
If you cancel for someone else, it’s not reflexive: Ik kan hem afmelden.

Where does zich go in the sentence? Can it move?

Default: place the reflexive pronoun early, before adverbials and before the infinitive: kan zich via de app tijdig afmelden.
You can insert adverbials between zich and afmelden (as in the sentence), but you cannot put zich after the verb: not ✗ kan afmelden zich.
Variants like kan zich tijdig via de app afmelden are also fine. Keeping the pronoun early usually sounds most natural.

Is the order via de app tijdig okay? Shouldn’t time usually come before manner?

Dutch often prefers Time–Manner–Place, but adverb order is flexible, especially with short, light adverbs like tijdig.
Both are fine: zich via de app tijdig afmelden and zich tijdig via de app afmelden. The given order is idiomatic.

What’s the nuance of tijdig compared with op tijd?

Tijdig = “in a timely manner” (slightly formal/administrative tone).
Op tijd = “on time” (more colloquial).
Meaning overlaps heavily; register is the main difference. Beware the false friend tijdelijk (“temporary”).

Why via de app and not in de app or met de app?

Via de app: through the app as a channel/method (most neutral for “use the app to do this”).
In de app: inside the app (focus on location/interface: “in the app, go to Settings”).
Met de app: possible but less idiomatic here; it can sound like “using the app as a tool,” which is usually covered by via.

Why is there a comma after Wie ziek is?

Because wie ziek is is a fronted subordinate (relative) clause functioning as the subject/topic. Dutch places a comma before the main clause in such cases: Wie ziek is, …
It aids readability and is standard punctuation.

Could I drop the modal and just use the present tense?

Yes: Wie ziek is, meldt zich via de app tijdig af.
That states it as a straightforward rule rather than an option/possibility.

Why die and not dat?

In the fixed correlative pattern wie … die …, the resumptive pronoun is die, not dat.
More generally, die is the common-gender/plural demonstrative and also the one used for people here; dat would be unidiomatic in this construction.

What about degene die instead of wie?

Singular: Degene die ziek is, kan zich via de app tijdig afmelden.
Plural: Diegenen/zij die ziek zijn, kunnen zich …
Degene is the standard form; diegene occurs in informal usage but is often discouraged in formal writing.

Is there a difference between zich afmelden and zich ziek melden?

Yes:

Zich afmelden = sign off/cancel participation or presence (e.g., for a class, appointment).
Zich ziek melden = report sick to an employer/school.
Depending on context, one or the other fits better. The sentence suggests cancelling an expected attendance via the app.

Do I need a preposition like voor or bij with afmelden?

Often you do when you name the target:

zich afmelden voor de les/het toernooi (for what you’re cancelling)
zich afmelden bij de receptie/HR (at/with whom you cancel) In the given sentence, the context of what/whom is understood, so it’s omitted.

How would I say this in the past or with other tenses?

Simple past: Wie ziek was, (die) kon zich via de app tijdig afmelden.
Present perfect: Wie ziek is geweest, (die) heeft zich via de app tijdig afgemeld.
With “to have to”: Wie ziek is, (die) moet zich via de app tijdig afmelden.

Why kan and not mag or moet?

Kan = can/is able/is allowed (neutral “this is possible”).
Mag = may/is permitted (emphasizes permission).
Moet = must/has to (obligation).
Official instructions often use kan to present an available option without imposing a requirement.