Learn Swedish: Pensionären byggde sig en ny bänk till verandan. - The pensioner built himself a new bench for the porch.

Here, sig acts as a benefactive indirect object, showing who benefits from the action. Just like in English ('built himself a bench'), it emphasizes that the pensioner built the bench for his own personal use. Leaving it out is grammatically fine, but loses that personal touch.

Could I use a preposition to say he built it for himself instead of using sig as the indirect object?

Yes! You could replace the indirect object sig with the prepositional phrase åt sig själv: Pensionären byggde en ny bänk åt sig själv. However, using åt places a much stronger, contrastive emphasis on the fact that he did it for himself and nobody else. The shorter byggde sig is more natural for a simple, relaxed statement.

Why do we use the preposition till instead of för to translate 'for the porch'?

In Swedish, till is typically used to indicate destination or where something belongs. Because the bench is physically destined to be placed on the porch, till is the correct choice. The preposition för is generally used when doing an action on someone's behalf, which doesn't make sense for an inanimate location.

If I start the sentence with a time phrase like Förra sommaren (Last summer), where does the reflexive pronoun sig go?