Learn Hardcore Dutch: Toen de monteur de reparatie uitvoerde, keek ik aandachtig mee. - When the mechanic carried out the repair, I watched attentively.

Use toen with past-tense verbs to describe one past moment.
Use wanneer for repeated or general occurrences (“Whenever I see him…”) or in questions (“Wanneer kom je?”).
Use als mainly for conditions (“If it rains…”) or repeated situations.

Why is the finite verb uitvoerde placed at the end of the clause Toen de monteur de reparatie uitvoerde?

In Dutch, subordinate clauses introduced by conjunctions (like toen, omdat, terwijl) follow the rule: conjunction + subject + objects + finite verb at the end.
So here:

toen (conjunction)
de monteur (subject)
de reparatie (object)
uitvoerde (finite verb)

Why is there a comma after uitvoerde, and how does that affect the next clause?

A comma separates the subordinate clause from the main clause. Because the sentence starts with a subordinate clause, the main clause undergoes inversion (verb–subject swap):

Subordinate clause: Toen … uitvoerde,
Main clause with inversion: keek ik in stead of ik keek.

How do the separable verbs uitvoeren and meekijken behave differently in the two clauses?

Both are separable verbs, but their prefixes attach or detach depending on clause type:

In the subordinate clause, the entire verb group moves to the end and the prefix reattaches: uitvoerde (not voerde … uit).
In the main clause, the prefix separates and goes to the very end: subject + finite verb + adverb + mee → keek ik aandachtig mee.

Why is aandachtig used here, and how do you form adverbs of manner in Dutch?

Dutch typically uses the adjective form directly as an adverb (no -ly ending).

aandachtig = “attentively” or “closely.”
It sits between the finite verb and the separable prefix in a main clause: keek (verb) + aandachtig (adverb) + mee (prefix).

Could we swap the clauses and say Ik keek aandachtig mee toen de monteur de reparatie uitvoerde? Would that still be correct?

Yes. If the main clause comes first, you drop the comma and keep normal word order (no inversion):

Ik keek aandachtig mee (S-V-Adv-Prefix)
toen de monteur de reparatie uitvoerde (subordinate clause with verb at the end)

Why is it de reparatie and not een reparatie or het reparatie?

de vs. het: Most nouns ending in -ie take de, and reparatie is one of them.
de vs. een: The definite article de signals that you mean that specific repair (the one the mechanic was doing), not just any repair.