Mal ist die Geldkassette voll, mal bleibt sie leer.

Here mal ... mal ... is a fixed pattern meaning sometimes ... sometimes ..., at one time ... at another time, or now ... now in a literary style.

It is used to show alternating situations:

• Mal ist die Geldkassette voll = sometimes the cash box is full
• mal bleibt sie leer = sometimes it stays empty

This mal is different from the everyday modal particle mal in phrases like Komm mal her.

Because German main clauses normally follow the verb-second rule.

That means:

• the first position can be taken by many different elements
• but the finite verb must come second

In Mal ist die Geldkassette voll, Mal takes the first position, so ist must come next.

The same thing happens in the second clause:

• mal = first position
• bleibt = second position
• sie = subject after the verb

Usually, mal ... mal ... is the more natural choice for this kind of contrast.

Manchmal simply means sometimes, but repeating it as manchmal ..., manchmal ... is less idiomatic when you want to present two alternating outcomes side by side.

So:

• Mal ist die Geldkassette voll, mal bleibt sie leer. = very natural
• Manchmal ist die Geldkassette voll, manchmal bleibt sie leer. = understandable, but less elegant in this pattern

Geldkassette is a compound noun:

• Geld = money
• Kassette = box, case, cash box

So die Geldkassette means something like cash box, cash tin, or money box, depending on context.

As with most German compounds, the last part decides the gender. Since die Kassette is feminine, the whole word is also feminine:

• die Geldkassette

Sie refers back to die Geldkassette.

Since Geldkassette is a feminine singular noun, the matching pronoun is sie:

• die Geldkassette → sie

So mal bleibt sie leer means sometimes it stays empty, where it refers to the cash box.

Bleibt leer means stays empty or remains empty, not just is empty.

That gives a slightly stronger nuance:

• ist leer = describes a state
• bleibt leer = emphasizes that it ends up empty or continues to be empty

So the sentence suggests not just two static conditions, but two different outcomes: sometimes money comes in, sometimes the cash box ends up empty.

Because here they are predicate adjectives, not adjectives directly before a noun.

After verbs like sein, bleiben, and werden, German adjectives normally do not take endings:

• Die Geldkassette ist voll.
• Die Geldkassette bleibt leer.

Compare that with adjectives before a noun, where endings are required:

• die volle Geldkassette
• die leere Geldkassette

It is in the nominative, because it is the subject of the clause.

In the first clause:

• die Geldkassette is the thing that is full

In the second clause, the subject is replaced by the pronoun sie, which also refers to that same nominative subject.

So this is not an object after ist or bleibt. With verbs like sein and bleiben, the subject stays in the nominative.

Because the sentence contains two coordinated main clauses:

• Mal ist die Geldkassette voll
• mal bleibt sie leer

German often separates such parallel clauses with a comma, especially when they are balanced and contrasted like this.

The comma helps show the rhythm and the contrast between the two alternatives.

It is capitalized because it comes at the beginning of the sentence.

In this use, mal is not a noun, so it would normally be written with a lowercase m inside a sentence:

• ..., mal bleibt sie leer.

So the capital M in Mal here is just normal sentence-initial capitalization.