Bevor ich einkaufe, schaue ich ins Gefrierfach, damit ich nichts doppelt kaufe.

Because einkaufen is a separable verb.

• In a normal main clause: Ich kaufe ein.
• In a subordinate clause: the verb goes to the end, and the prefix stays attached: bevor ich einkaufe

So bevor ich einkaufe is exactly what you expect after a word like bevor.

German main clauses follow the verb-second rule. The whole clause Bevor ich einkaufe takes the first position in the sentence, so the finite verb of the main clause must come next:

Bevor ich einkaufe, schaue ich ins Gefrierfach ...

So this is not unusual word order at all. It is the normal result of putting the bevor clause first.

bevor means before and introduces a subordinate clause.

That matters because in German subordinate clauses, the conjugated verb usually goes to the end:

• bevor ich einkaufe
• not bevor ich kaufe ein

This is the same general pattern you see with words like weil, dass, and damit.

Because German normally uses commas to separate subordinate clauses from the main clause.

Here you have:

• Bevor ich einkaufe, → subordinate clause
• schaue ich ins Gefrierfach, → main clause
• damit ich nichts doppelt kaufe. → subordinate clause

So both commas are required.

They are related, but not identical.

• einkaufen = to go shopping / do the shopping
• kaufen = to buy

So:

• Bevor ich einkaufe = Before I go shopping
• ... damit ich nichts doppelt kaufe = ... so that I don’t buy anything twice

Using both verbs in the same sentence is completely natural.

ins is just the contraction of in das.

• in das Gefrierfach → ins Gefrierfach

After schauen in the sense of look into, German commonly uses in + accusative, so ins fits naturally here.

Gefrierfach means freezer compartment.

It usually refers to the freezing section inside a fridge or a small freezer section. It is a very everyday household word.

Related words:

• Gefrierschrank = freezer
• Tiefkühltruhe = chest freezer

Here damit means so that or in order that.

It introduces a purpose clause: it explains why the speaker looks in the freezer.

So:

damit ich nichts doppelt kaufe
= so that I don’t buy anything twice

And because damit introduces a subordinate clause, the verb goes to the end again: kaufe.

Because nichts is the object: nothing / anything.

The idea is:

• ich kaufe nichts doppelt = I don’t buy anything twice

Using nicht instead would negate something different and would sound less natural here. In this sentence, the speaker is saying they want to avoid buying any item twice, so nichts is the natural choice.

Here doppelt means twice or as a duplicate.

So nichts doppelt kaufen means:

• not buy the same thing twice
• not buy duplicates

It does not mean buy double the amount here.

Because each clause has its own subject.

• Bevor ich einkaufe
• schaue ich ins Gefrierfach
• damit ich nichts doppelt kaufe

German normally states the subject separately in each full clause, even when it is the same person each time.

Because German very often uses the present tense for future meaning when the context already makes the timing clear.

Here the sentence structure already shows the sequence:

• first: check the freezer
• later: go shopping

So einkaufe, schaue, and kaufe are all present tense forms, but the meaning is naturally understood as present/future or habitual.

Yes. Bevor ich einkaufen gehe is also correct and very common.

There is a slight difference in feel:

• Bevor ich einkaufe = before I shop / before I do the shopping
• Bevor ich einkaufen gehe = before I go shopping

In everyday speech, both are natural. The version in your sentence is a little more compact.