It seems like you can use only the indefinite article with this phrase, but I don't get it. You can say: "in a past life", but not "in the past life" though you can say "in the past year". What's the rule for this? Thanks in advance!
The definite article 'the' identifies something that the listener has already heard mentioned. There are many 'past years', and perhaps also 'past lives'. 'The past year' implies the most recent 12 months, but unless you are deep into a conversation about specific past lives, 'the [most recently] past life' won't bring any details to mind in the listener.
You can say in the past life in the right context.
By itself, the phrase is an idiom, typically used to introduce a description of something that was experienced in a different context, e.g.
Alphonse: I'm actually enjoying this hike. In a past life I must have been an explorer.
Gaston: Really? In the past life someone drove your luggage ahead to the hotel?
Used as an introduction, in a past life can be linked to several ideas, e.g. reincarnation where you repeat something you have done before, or reincarnation with karma where you rise or sink in your next life depending on how you behaved in the previous one. These are concepts that few educated people take literally, so in a past life usually sets up the reader or listener for some kind of ironic or humorous observation.
Once you're past its introduction, though, you can refer to the past life exactly as you would any other noun, e.g. "I have a wallet. The inside of the wallet is divided into two parts. It's my favorite wallet, etc."
In contrast, in the past year is more specific and carries the connotation that we are already in the present year, so the context is established and you can use the definite article right at the start.