I came across these two points on bunpro and wonder if some things are left unexplained. I’m not sure how to parse these.
The first one is [past form] + 方 がいい. I understand the がいい as a way to say “would be good / better”, similar to " - てもいい". But I’m not sure what 方 is here. Is it a nominalizing particle ?
Then there is [negative without the い] なくてはいけない. This means something must be done. I get the double negation, but I’m not sure what the て-form and は are doing. Is it the は particle, and could I replace it with が if I’m already using it somewhere else ?
For some reason, I feel these two expressions could be using the same grammatical structure, but don’t.
No, 方 isn’t a particle, it’s just a noun. In this kind of context it’s indicating one side of a comparison or choice, and it’s the same as in 黒より白のほうがよい and so on.
In なくてはいけない, the は is particle-は (you can tell by the pronunciation). You get は in this pattern and が in the other for the usual reasons you tend to get get が in positive and は in negative sentences generally. Don’t try to mess around with the structure, they’re pretty fixed phrases.
The ては is a special case of a more general grammar pattern that doesn’t come up til later (it’s in the Dictionary of Intermediate Japanese, for instance, which says it “presents an action/state as a topic about which a negative comment is given”). Don’t worry about that for now, just treat these as fixed patterns.
The 方 in ～た方がいい means “way” or “direction,” which, in this case, would be “way of doing.” There’s a separate grammar point on Bunpro about how it can be used, but you’re gonna get to that soon if you haven’t seen it already.
～た方がいい is basically saying “(this) way of doing is good.”
You’re familiar with the vocab 仕方がない, yeah? That means “can’t be helped,” but a more literal translation could be “way of doing does not exist (which is why it can’t be helped).”
For なくてはいけない, I’m not familiar enough with the grammar to give a satisfying answer. The way I see it, the は is just marking the sentence topic, same as usual. It might help to think of it as three separate segments (EG: 分からなくてはいけない = “not understand” is “must not” = “must understand”).