Vor der Reise packe ich nur das Nötigste in den Rucksack.

I

Breakdown of Vor der Reise packe ich nur das Nötigste in den Rucksack.

• in: into
• nur: only
• die Reise: the trip

Questions & Answers about Vor der Reise packe ich nur das Nötigste in den Rucksack.

Why is it Vor der Reise and not Vor die Reise?

Because the preposition vor takes the dative case when it means a static time/location sense like before (time) or in front of (place).

• die Reise (nominative) → der Reise (dative)
  So vor der Reise = before the trip.
  (Vor can also take accusative in directional/“to a position in front of” uses, but not in this time meaning.)

Why does the sentence start with Vor der Reise? Is the word order flexible?

Yes. German often puts a time phrase first for emphasis or flow. The key rule is: the finite verb stays in position 2 in a normal main clause.
So if you put Vor der Reise in position 1, the verb packe must stay in position 2, and the subject moves after it:

• Vor der Reise (1) packe (2) ich (3) …

You could also say:

• Ich packe vor der Reise nur das Nötigste in den Rucksack.

Why is it packe ich instead of ich packe?