Swedish: An Essential Grammar, 2nd edition

Sin/sitt/sina cannot be used to qualify the subject of the clause:
His wife loves him.
Hans fru älskar honom.
S
2 The non-reflexive forms do not refer back to the subject of the clause:
Olle is cross. Why? Because Åke went out with his wife.
Olle är sur. Varför det? Därför att Åke gick ut med hans fru.
S
S
O
The non-reflexive possessive pronouns hans, hennes, dess, deras
(indeclinable) may qualify the subject or object of a clause:
His wife is a teacher.
I’ve seen his wife at school.
Hans fru är lärare.
Jag har sett hans fru på skolan.
S
O
There are two ways of maintaining a distinction between the areas of
usage of these different forms:
(a) Draw an arrow to the ‘possessor’. Is the ‘possessor’ the subject of
the clause?
(b) Can you insert the word ‘own’ before the object in English? If so,
use a form of sin/sitt/sina. If not, then use a non-reflexive form.
3 A problem arises when there is more than one clause in the sentence:
They think that their teacher is boring.
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/SC S
Here ‘their teacher’ is the subject of the subordinate clause and ‘their’
does not refer back, but qualifies the subject. Therefore use deras.