Learn Hardcore German: Bitte melde dich nicht ab, solange die Prüfung läuft. - Please don’t log off as long as the exam is in progress.

The infinitive is sich abmelden.
For du (informal singular you), the imperative is built from the ich-form without the -e:
- ich melde → imperative du: meld(e)!
In the full sentence, that becomes:
- Melde dich nicht ab. / Meld dich nicht ab.

So melde is the imperative form, while meldest is the normal present tense form (du meldest dich ab = you sign off / log out).

What does dich refer to, and why is the verb reflexive?

The verb is reflexive: sich abmelden. That literally means to deregister oneself, and it’s simply how German expresses to sign off / to log out / to check out in many contexts.

sich abmelden needs a reflexive pronoun.
For du, the accusative reflexive pronoun is dich.

So:

du meldest dich ab = you sign yourself off / you log out
Bitte melde dich nicht ab = Please don’t log out (yourself).

Even though English doesn’t use yourself here, German typically uses the reflexive pronoun with many verbs where English doesn’t.

What is the role of ab here, and why does it come at the end?

Abmelden is a separable-prefix verb:

ab- is a separable prefix.
The base verb is melden (to report / register).
abmelden = to deregister, to sign off, to log off/out.

In main clauses and in the imperative, the prefix ab is separated and placed at the end of the clause:

Present: Ich melde mich ab. (I’m logging out.)
Imperative: Melde dich nicht ab.

So ab goes to the end of the clause because of the standard word-order rule for separable-prefix verbs in German main clauses and imperatives.

Is Bitte melde dich nicht ab formal or informal? How would I say it formally?

Bitte melde dich nicht ab is informal, because it uses the du form (melde, dich).

To say this formally to one person (using Sie):

Bitte melden Sie sich nicht ab, solange die Prüfung läuft.

To say it to more than one person informally (plural ihr):

Bitte meldet euch nicht ab, solange die Prüfung läuft.

To say it to more than one person formally (plural Sie, same as singular):

Bitte melden Sie sich nicht ab, solange die Prüfung läuft.
(Same form as the singular polite.)

Could you also say Meld dich nicht ab instead of Melde dich nicht ab?

Yes. In everyday spoken German, the -e at the end of the du-imperative is often dropped:

Melde dich nicht ab. (more complete/formal)
Meld dich nicht ab. (very common in speech, also fine in informal writing)

Both are correct. Meld dich nicht ab sounds a bit more colloquial and natural in casual conversation.

Why is there a comma before solange?

Solange introduces a subordinate clause (a dependent clause) that expresses a condition that lasts over time: as long as / while.

German spelling rules require a comma before most subordinate clauses, regardless of length. So you must write:

Bitte melde dich nicht ab, solange die Prüfung läuft.

The part starting with solange is its own clause, hence the comma.

Why is the verb läuft at the end of the clause solange die Prüfung läuft?

Because solange introduces a subordinate clause, and in German subordinate clauses the finite verb goes to the end.

Word order:

solange (subordinating conjunction)
die Prüfung (subject)
läuft (verb at the end)

So the clause structure is:

solange
- subject + rest + verb
  → solange die Prüfung läuft

This is the standard pattern with subordinating conjunctions in German (e.g. weil, dass, wenn, obwohl, solange, etc.).

What does solange mean exactly, and how is it different from während or so lange?

Solange is a subordinating conjunction that means “as long as” (in a temporal, during the time that sense, often with an implicit condition):

Solange die Prüfung läuft, melde dich nicht ab.
As long as the exam is in progress, don’t log out.

Differences:

solange (one word)
- Conjunction: as long as (during the time that).
- Always followed by a clause with verb at the end.
während
- Conjunction: while, whereas.
- Here: Während die Prüfung läuft, melde dich nicht ab. = While the exam is in progress…
- Often similar to solange, but während is more neutral “while”, whereas solange can more strongly imply “for the entire duration that X is true”.
so lange (two words)
- Adverb + adjective: so long, for such a long time.
  Example: Ich kann nicht so lange warten. = I can’t wait that long.
- Not a conjunction; it doesn’t introduce a clause.

In your sentence, you need the conjunction meaning as long as → solange (one word).

Why do you say die Prüfung läuft instead of something like die Prüfung ist?

In German, laufen is often used for events, processes, or activities that are taking place or underway:

Der Film läuft. – The movie is playing.
Die Sitzung läuft. – The meeting is in progress.
Der Vertrag läuft noch. – The contract is still valid / ongoing.

So die Prüfung läuft means the exam is in progress / is currently happening.

Using sein (ist) here would sound incomplete:
die Prüfung ist → the exam is … (what?) It needs a complement (e.g. ist schwer, ist heute, ist vorbei).

You could rephrase with a different verb:

solange die Prüfung noch dauert – as long as the exam is still going on

But die Prüfung läuft is a very idiomatic and natural expression.

In an online context, could I say Bitte logge dich nicht aus instead? Is there any difference from melde dich nicht ab?

Yes, in many computer / online contexts you can say:

Bitte logge dich nicht aus, solange die Prüfung läuft.

Differences in nuance:

sich abmelden
- More general: to sign off, check out, deregister, log out.
- Used both in digital and non-digital contexts (e.g. sich vom Kurs abmelden = to withdraw from a course).
sich ausloggen / sich einloggen
- Borrowed from English to log out / log in.
- Very common in purely digital contexts.

In an online exam, both melde dich nicht ab and logge dich nicht aus can be understood as don’t log out. Some platforms specifically use abmelden on their buttons, so the sentence with abmelden matches that wording more closely.