Learn Swedish: Då fyren är gammal, är skenet svagt. - Since the lighthouse is old, the glow is weak.

While eftersom is the most common word for "because" or "since", då is also frequently used as a causal conjunction, especially in slightly more formal or written Swedish. They are grammatically interchangeable here, but recognizing and using då adds a nice variety to your vocabulary at the B2 level.

Doesn't då usually mean "then" or "at that time"? How do I know it means "since" here?

Context is key! While då often acts as an adverb meaning "then" (like in vi ses då), it also works as a subordinating conjunction. When it starts a clause in the present tense explaining a reason, like Då fyren är gammal, it almost always means "since" or "because". If the sentence strictly meant "when", a Swede would normally use när.

Why is the word order flipped to är skenet svagt instead of skenet är svagt?

This is the classic V2 (verb-second) rule in action. The entire subordinate clause (Då fyren är gammal) counts as the first "position" or block of information in the main sentence. To keep the main verb in the second position, the verb (är) must come right after the comma, pushing the subject (skenet) to the third position.

Why does the adjective svagt get a -t ending here even though it is placed after the noun?