Learn Danish: Vi sparer op når vi kan.

Når is used for things that happen in the present or future, and for things that happen repeatedly. Because 'we save up when we can' is a general, ongoing habit in the present tense, we must use når. You would only use da for a single, specific event in the past.

What is the difference between sparer and sparer op?

Sparer on its own means saving in a general sense, like saving time, saving energy, or cutting back on expenses. Adding op makes it the phrasal verb at spare op, which specifically means to save up money over time.

Why does the word order stay as vi kan after når?

Because når vi kan is a subordinate clause, which is a dependent part of the sentence introduced by når. In Danish subordinate clauses, the subject always comes right after the conjunction, followed by the verb. Therefore, we keep the straight word order vi kan and do not invert the verb.

Is it normal to end the sentence with kan without repeating the main verb?

Yes, exactly like in English. If the main action is already obvious from the first part of the sentence, you can just end the clause with the modal verb kan. You do not need to repeat the action by saying når vi kan spare op.