Bitte klapp die Brotdose zu, sonst bleibt das Brot nicht frisch.

Because the full verb is zuklappen, which is a separable verb.

• Infinitive: zuklappen
• In a main clause, the prefix zu- moves to the end:
  • Ich klappe die Brotdose zu.
  • Bitte klapp die Brotdose zu.

This is very common in German with verbs like:

• aufmachen → Mach das Fenster auf.
• zumachen → Mach die Tür zu.
• einkaufen → Ich kaufe heute ein.

So here, klapp ... zu is just the imperative form of zuklappen.

This is the informal singular imperative used with du.

For many verbs, the du command is formed from the stem:

• kommen → Komm!
• machen → Mach!
• klappen / zuklappen → Klapp zu!

You may also sometimes hear klappe zu, but klapp zu is the more usual everyday command form.

So Bitte klapp die Brotdose zu is speaking to one person informally.

Die Brotdose is the direct object of the verb, so it is in the accusative case.

The noun Brotdose is feminine:

• nominative: die Brotdose
• accusative: die Brotdose

For feminine nouns, nominative and accusative look the same, so the article stays die.

Compare:

• die Dose → nominative
• Ich schließe die Dose. → accusative

So the form does not change here, even though it is the object.

Brotdose is a compound noun:

• Brot = bread
• Dose = container, box, tin

Together, Brotdose usually means a lunchbox, sandwich box, or food container, especially one used for taking bread or snacks with you.

German often builds nouns this way, and the last part determines the gender:

• die Dose
• therefore die Brotdose

Zuklappen usually means to fold shut, snap shut, or close a hinged thing.

It is especially natural for things like:

• a lunchbox
• a laptop
• a lid that flips shut

Related verbs:

• schließen = to close, more neutral or formal
• zumachen = to close, very common and everyday
• zuklappen = to close by folding/snapping shut

So with a Brotdose, zuklappen sounds very natural because many lunchboxes have a lid you flip closed.

Because German main clauses follow the verb-second rule.

When sonst comes first, the conjugated verb must come next:

• Sonst bleibt das Brot nicht frisch.

Structure:

• sonst = first position
• bleibt = second position
• das Brot = after the verb

This is the same pattern as:

• Heute gehe ich nach Hause.
• Dann kommt er später.
• Sonst wird es teuer.

So sonst takes the first slot, and that forces bleibt to come before the subject.

Here sonst means otherwise.

So the sentence means something like:

• Please close the lunchbox, otherwise the bread won’t stay fresh.

It introduces a consequence if the command is not followed.

Common examples:

• Beeil dich, sonst verpassen wir den Zug.
• Zieh eine Jacke an, sonst wird dir kalt.

Because the idea is stays fresh, not becomes fresh.

• bleiben = to remain, stay
• werden = to become

So:

• Das Brot bleibt frisch. = The bread stays fresh.
• Das Brot wird frisch. = The bread becomes fresh.

The second one does not fit the meaning here. The bread is fresh now, and closing the lunchbox helps it remain fresh.

Because frisch is being used predicatively, not directly before a noun.

After verbs like:

• sein
• werden
• bleiben

adjectives usually do not take endings.

So:

• das frische Brot = the fresh bread
  
  • adjective before a noun, so it gets an ending
• Das Brot bleibt frisch.
  
  • adjective after bleibt, so no ending

This is the same in:

• Das Wasser ist kalt.
• Er wird müde.
• Die Tür bleibt offen.

German can absolutely use es here:

• Bitte klapp die Brotdose zu, sonst bleibt es nicht frisch.

But repeating das Brot sounds clearer and more explicit, especially in everyday speech.

Using the noun again can help avoid even small ambiguity, because es could in theory refer to something else from the context. So repeating das Brot is completely natural.

Yes. Adding bitte makes the imperative softer and more polite.

Without bitte:

• Klapp die Brotdose zu.
  This is more direct.

With bitte:

• Bitte klapp die Brotdose zu.
  This sounds like a normal polite request to someone you know well.

It is still informal because it uses the du imperative. If you wanted to address someone formally, you would say:

• Bitte klappen Sie die Brotdose zu.

So the sentence is polite, but informal.