Learn Hardcore German: Wir sortieren die Kiste gemeinsam und bewahren die Briefe im Schrank auf. - We sort the box together and keep the letters in the cupboard.

Because aufbewahren is a separable-prefix verb. In main clauses, the finite verb stays in second position and the separable prefix goes to the end of its clause: … bewahren … auf.

Main clause: Wir bewahren die Briefe im Schrank auf.
Subordinate clause: …, dass wir die Briefe im Schrank aufbewahren. (no split)
Imperative: Bewahre die Briefe im Schrank auf!
Perfect: Wir haben die Briefe im Schrank aufbewahrt. (prefix stays attached in the participle)

What is going on with und and the verb position in the second half?

Und starts a new main clause, so verb‑second (V2) applies again. The subject wir is understood from the first clause and can be omitted, so you get … und bewahren … auf. You could also repeat the subject: … und wir bewahren die Briefe im Schrank auf. Both are correct; omitting it is more concise when the subject doesn’t change.

What does im stand for?

im is the contraction of in dem (in + the, dative). Similarly:

ins = in das (in + the, accusative, neuter)
am = an dem Here, Schrank is masculine, so the dative is dem Schrank → im Schrank.

Why im Schrank and not in den Schrank?

Because in is a two‑way preposition:

Dative (location/state): im Schrank = in the cupboard (where it is).
Accusative (motion/direction): in den Schrank = into the cupboard (where it goes). Compare:
Wir bewahren die Briefe im Schrank auf. (they’re kept there)
Wir legen die Briefe in den Schrank. (we put them there)

Why are both die Kiste and die Briefe using die?

Different reasons:

die Kiste: feminine singular in the accusative (direct object of sortieren).
die Briefe: plural accusative (direct object of aufbewahren). In the plural, nominative and accusative both use die for all genders. Note: Singular → der Brief, plural → die Briefe.

Could it be der Kiste or den Briefen instead? When do those forms appear?

Those are dative forms.

der Kiste (dative singular): after dative prepositions or for location with two‑way preps: mit der Kiste, in der Kiste.
den Briefen (dative plural, note the -n): mit den Briefen, zu den Briefen; also as an indirect object with verbs that take dative: Wir widmen den Briefen viel Zeit.

What exactly does aufbewahren mean compared to behalten, lagern, or aufheben?

aufbewahren: store/keep safe in a particular place, often for some time. Wir bewahren die Briefe im Schrank auf.
behalten: keep/retain (possession or decision), not about where you store it. Ich behalte die Briefe.
lagern: store goods in stock/warehouse; larger‑scale or commercial. Die Firma lagert die Ware.
aufheben: colloquial for keep/save, but also “pick up” and “nullify/abolish” in formal contexts. Heb den Kassenzettel auf. Also, bewahren alone means “to preserve/maintain”: die Ruhe bewahren.

Can I move gemeinsam to other positions? For example, Wir sortieren gemeinsam die Kiste?

Yes. Gemeinsam (manner adverb) is flexible:

Neutral: Wir sortieren die Kiste gemeinsam.
Slight emphasis on togetherness: Wir sortieren gemeinsam die Kiste.
Strong emphasis (fronting): Gemeinsam sortieren wir die Kiste. Just keep the finite verb in second position.

Is there a difference between gemeinsam and zusammen?

Often interchangeable for “together,” but:

gemeinsam emphasizes joint action/shared responsibility; a bit more formal.
zusammen emphasizes being together in time/space; very common colloquially. Your sentence works fine with either: Wir sortieren die Kiste zusammen …

Do Germans really say die Kiste sortieren? In English you’d sort the contents, not the box.

Yes. Die Kiste sortieren is idiomatic and usually understood as “sort the box’s contents” or “sort through the box.” If you want to be explicit:

Wir sortieren den Inhalt der Kiste.
Wir sortieren die Briefe aus der Kiste.
Wir sortieren die Kiste durch. (sort through the box) Be careful: die Kiste aussortieren means removing/dispensing with the box (or its contents) from a selection.

Why is there no comma before und here?

German normally does not use a comma before und when linking main clauses. A comma can be optional for readability or to avoid ambiguity, but the standard here is without: Wir sortieren … und bewahren … auf.

Can I say Wir bewahren im Schrank die Briefe auf? Where does the place phrase go?

It’s possible, but the neutral order places the direct object before the place phrase: … die Briefe im Schrank …. Moving im Schrank earlier gives it focus. Some tendencies in the middle field:

Pronouns usually come before full noun phrases: Wir bewahren sie im Schrank auf.
Location often comes toward the end, especially after the direct object.

Why are the nouns Kiste, Briefe, Schrank capitalized?

All German nouns are capitalized. Verbs, adjectives, and most pronouns are not (except formal Sie and its possessive Ihr). Wir is capitalized here because it starts the sentence.

Can the present tense here also refer to the future?

Yes. German often uses the present for near‑future plans if context makes the time clear. Future with werden is possible but not required:

Wir sortieren die Kiste morgen und bewahren die Briefe im Schrank auf.
Wir werden die Kiste morgen sortieren und die Briefe im Schrank aufbewahren.

Pronunciation tips for key words?

Briefe: ie = long “ee”; roughly BREE-feh.
gemeinsam: ei = “eye”; roughly geh-MINE-zahm (hard g).
auf/aufbewahren: au = “ow” in cow; w sounds like English v; roughly owf-beh-VAH-ren.
Schrank: sch = “sh”; roughly shrahngk. Final consonants may sound a bit devoiced.

Could I drop the article and say … und bewahren Briefe im Schrank auf?

Grammatically yes, but the meaning changes. die Briefe refers to specific, known letters. Bare plural Briefe means some letters in general. Use the article if you mean those particular letters you’ve just sorted.