Learn Swedish: Fyrvaktaren trivdes bra, då isoleringen gav honom lugn och ro. - The lighthouse keeper thrived, since the isolation gave him peace and quiet.

In this sentence, då acts as a subordinating conjunction meaning 'since' or 'because', introducing a subordinate clause. It means exactly the same thing as eftersom, but då can sound slightly more formal or literary. Remember that since it introduces a subordinate clause, any clausal adverb like inte would come before the verb (for example, då isoleringen inte gav honom...).

Why do we say trivdes bra instead of just trivdes?

While in English you can simply say someone 'thrived' or 'enjoyed themselves', the Swedish verb trivas usually expects an adverb like bra (well) or dåligt (poorly) to complete the thought. Saying Fyrvaktaren trivdes on its own would sound a bit abrupt and incomplete to a Swedish ear.

Does the -s on trivdes mean this is a passive sentence?

No, trivas is a deponent verb. These are verbs that always end in -s in all their forms but have an active meaning. The past tense of trivas is trivdes. Other common deponent verbs you likely already know are hoppas (to hope) and minnas (to remember).

Is lugn och ro a fixed phrase?