Learn Icelandic: Mágur minn ætlar að bólstra stólinn. - My brother-in-law is going to upholster the chair.

This sentence shows the direct action, meaning the brother-in-law is doing the upholstering himself. If he were paying someone else to do it, you would use the causative structure you are currently learning: Mágur minn ætlar að fá stólinn bólstraðan (My brother-in-law is going to get the chair upholstered).

Why is it spelled stólinn with one L here, instead of stóllinn?

The verb bólstra (to upholster) takes an accusative object. The dictionary form (nominative) of the word for 'the chair' is stóllinn with two Ls, but its accusative form drops one L to become stólinn. This is a common spelling change for masculine nouns ending in -ll when they shift to the accusative case.

Does mágur cover all types of brothers-in-law?

Yes, mágur translates to brother-in-law and covers both your sister's husband and your spouse's brother. It is one of the more straightforward kinship terms in Icelandic, which otherwise often demands very specific words for maternal versus paternal relatives.

Why does the possessive word minn come after the noun instead of before it?

In Icelandic, possessive pronouns like minn (my) normally follow the noun. Saying minn mágur is not grammatically wrong, but it puts a heavy, unnatural emphasis on the word 'my' and sounds overly poetic or dramatic for everyday speech.