"He loves his wife."
Translation:Han elsker kona si.
sin used with masculine (en) words.
Si used with feminine (ei) words. (Feminine words are rare, and the can also be used in the masculine. So ei jente and en jente are both correct. Usually words ending in -a in the def. sing. - jenta, kona - are feminine.)
Sitt used with neutral (et) words.
Sine used with plurals.
Sin/si/sitt/sine are used when: 1) The subject is a third person noun (he/she/it/they/Someone's name) 2) The possessive is referring back to the subject.
"He loves his car" - "Han elsker bilen sin." (subject is third, possessive refers to subject) "I love his car" - "Jeg elsker bilen hans" (subject is first not third, possessive refers to another person)
Hope this helps.
Edit: Added "someone's name" to point out they don't have to be pronouns
In vestlandet it is used (not so much for kona, but almost everywhere else). I am actually confused when I'm somewhere else and I encounter the femininum. And this is actually not a new development, judging from street signed etc.
Strictly speaking what you call masculinum is actually the utrum, meaning both non-neutrum grammatical genders have the same form... Yes, it is similar to the male (masculinum) form.