「が」vs「の」 with possessives

Question

I know that the normal possessive form is usually formed subject+「の」+object. Though, in one instance, I found が being used in 天は我が物. I know that 「が」 can be used to express possession, though is there a certain way it can be used.

Answer 1

が for possession was more common in old Japanese.

But it's rare today and it only remains in proverbs (e.g. [人間]{にんげん}[万事]{ばんじ}[塞翁]{さいおう}が[馬]{うま}) and other fixed phrases.

One exception is [我]{わ}が. Usages as follows is common today.

• 我が社, 我が国, 我が母校, etc.
• 我が物顔
• [我]{わ}が[家]{や}

我が is still old-fashioned has a bit arrogant nuance, so if someone is using 我が unlimitedly, I feel like he is playing a role of [魔王]{まおう}.

I recommend you to use が for possession only in such fixed phrases.

Answer 2

The possessive が is literary and archaic, and I don't think it's productive anymore. You'll rarely see it outside several fixed expressions such as:

• [我]{わ}がＸ (e.g. 我が[家]{や}、我が子、我が国、我が道、我が命、我が物(にする)、我が物顔(で)...)
• [我]{われ}らがX (e.g. 我らが母校、我らが祖国、我らがヒーロー)
  (Not 私がＸ or 私たちがＸ, since the が is literary but 私・私たち are not.)
• 『君が代』
• Xがために (=Xのために) (e.g. [誰]{た}がために、これがために、～せんがために)
• Xが如く (=Xの如く)