Wanneer de garage open is, ben ik daar aan het sleutelen aan mijn auto.

When the garage is open, I am working on my car there.

Breakdown of Wanneer de garage open is, ben ik daar aan het sleutelen aan mijn auto.

• ik: I
• zijn: to be
• open: open
• mijn: my
• daar: there
• wanneer: when
• aan: on
• de auto: the car
• aan: at
• de garage: the garage
• het sleutelen: the tinkering

Questions & Answers about Wanneer de garage open is, ben ik daar aan het sleutelen aan mijn auto.

Why do we start the sentence with wanneer? Can we use als instead?

wanneer is a temporal conjunction meaning “when” or “whenever.” You can often swap it with als in Dutch when you mean “whenever” or “once,” but there’s a slight nuance:

• wanneer often stresses the timing (“at the moment that…”).
• als can also mean “if,” so in some contexts it could be ambiguous.

In this sentence wanneer clearly marks a recurring condition (“whenever the garage is open…”).

Why is the verb is at the very end of the first clause (wanneer de garage open is)?

Because clauses introduced by subordinating conjunctions like wanneer use verb-final word order. In those subordinate clauses the finite verb goes to the end:
…wanneer de garage open is.

In the second part we have ben ik daar instead of ik ben daar. Why does the verb come before the subject here?

After a fronted element (here the entire wanneer-clause), the main clause undergoes inversion: the finite verb comes first, then the subject. So you get:
wanneer …, ben ik daar…

What is aan het sleutelen, and how does that differ from just sleutelen?

aan het + infinitive is the Dutch way to express a continuous or progressive action (like English “to be …-ing”).

• aan het sleutelen = “tinkering/working on” right now.
  If you just say ik sleutel, it sounds more like a simple habitual action (“I tinker [in general]”).

Why are there two instances of aan in ben ik daar aan het sleutelen aan mijn auto?

They have different functions:
1) The first aan is part of the progressive construction aan het sleutelen.
2) The second aan is the preposition that links the verb “sleutelen” to its object “mijn auto” (“to work on my car”).

What role does daar play in this sentence, and can I omit it?

daar refers back to the garage (the place where you’re working). It adds a locational emphasis: “I’m over there.” You can omit it:
wanneer de garage open is, ben ik aan het sleutelen aan mijn auto.
But including daar makes the location explicit and sounds more natural in spoken Dutch.

Could I express the same idea without the progressive aan het construction?

Yes. A more neutral way is:
Wanneer de garage open is, werk ik aan mijn auto.
Here werk ik aan mijn auto uses the simple present (“I work on my car”) and still conveys that you go there to do that whenever it’s open.