Learn Hardcore German: Wenn der Fortschrittsbalken voll ist, erscheint oben ein Stern als Symbol für den Tageserfolg. - When the progress bar is full, a star appears at the top as a symbol of the day's success.

Main clause: Der Fortschrittsbalken ist voll. (verb in 2nd position)
Subordinate clause: … wenn der Fortschrittsbalken voll ist. (verb at the end)

So:

Wenn (subordinating conjunction)
der Fortschrittsbalken (subject)
voll (predicate adjective)
ist (verb at the end)

This is standard verb-final word order in German subordinate clauses.

Why is there a comma before erscheint?

German always separates a subordinate clause from the main clause with a comma.

Subordinate clause: Wenn der Fortschrittsbalken voll ist,
Main clause: erscheint oben ein Stern als Symbol für den Tageserfolg.

Rule: A subordinating conjunction like wenn, weil, dass, obwohl etc. introduces a subordinate clause, and that clause is separated with a comma from the main clause.

So the comma is mandatory here in correct German spelling.

Why is the word order erscheint oben ein Stern and not Ein Stern erscheint oben?

Both are possible and grammatically correct:

Erscheint oben ein Stern …
Ein Stern erscheint oben …

German main clauses follow the rule: the conjugated verb must be in second position (the “V2 rule”). What can be in first position is flexible: subject, adverb, object, etc.

In the sentence:

erscheint = verb (must be 2nd)
oben = adverbial (here it occupies the “first position” slot)
ein Stern = subject

So the structure is:

oben (position 1 – adverb)
erscheint (position 2 – verb)
ein Stern (rest of the sentence – subject)

This order emphasizes oben (“at the top”), i.e., the location, which is often what the UI text wants to stress.

If you say Ein Stern erscheint oben, you’re emphasizing ein Stern more than the location. Nuance, not correctness, is the difference.

What does wenn mean here? Is it “if” or “when”?

In this context, wenn means something like “when(ever)”:

Wenn der Fortschrittsbalken voll ist …
→ Whenever / When the progress bar is full …

It expresses a general condition that can happen repeatedly, not a one‑time event. English could use when or if here, depending on style.

German wenn can mean:

if (conditional): Wenn es regnet, bleibe ich zu Hause.
when/whenever (repeated situation): Wenn ich Zeit habe, lese ich.

To emphasize a one-time past event, German would use als instead:
Als der Fortschrittsbalken voll war, erschien oben ein Stern.
(When the progress bar was full [that one time]…)

Why is erscheint in the present tense if this talks about a future event (“when it is full, a star will appear”)?

German often uses the present tense for future situations when the context makes the time clear.

So:

Wenn der Fortschrittsbalken voll ist, erscheint oben ein Stern.

literally is in the present, but it’s understood as:

When the progress bar becomes full, a star appears (will appear) at the top.

You could use future tense:

… wird oben ein Stern erscheinen.

But it sounds more formal and less natural in this kind of UI/help text. Present tense for future conditions is completely normal in German.

What is the subject in erscheint oben ein Stern als Symbol für den Tageserfolg?

The subject is ein Stern.

The word order is:

erscheint (verb)
oben (adverbial – “at the top”)
ein Stern (subject)
als Symbol für den Tageserfolg (adverbial phrase explaining in what role the star appears)

Even though ein Stern comes after oben, it is still the subject because it is the thing that “appears.” German doesn’t rely on position to mark subject as strongly as English; instead, it uses case endings (here: nominative ein Stern, masculine singular).

Why is it der Fortschrittsbalken and not den Fortschrittsbalken or something else?

Fortschrittsbalken is the subject of the subordinate clause:

der Fortschrittsbalken = subject (nominative case)
ist = verb
voll = predicate adjective

In German, the subject takes the nominative case. For masculine singular nouns, the nominative article is der.