I'm not sure if my explanation will be helpful, but consider that the particle で indicates not just place and time, but also usage or manner. It is for this reason that Tae Kim refers to it as a "context particle". As such, in the phrase 「これでいい？」 , the particle gives the phrase a sense of confirmation if something is being done satisfactorily. It translates to "Is this fine?", or, more wordily, "Is the way this is like fine?". Thus, これ basically acts like a pronoun (that is, you can replace it with a noun or noun phrase), and で is pointing out its function in a phrase*. Think of で as a helpful little dude who's telling you what これ is and what it's doing there.

Hope that cleared things up. If anyone finds any mistakes in my explanation, please don't hesistate to point it out. ^^

*Have you noticed how particles make the syntax in Japanese sentences much clearer? I know it's completely unrelated, but I find that fascinating.

Metaler
Answered 5 years ago

Metaler-san,

Great explanation!! I just want to add the nuance that the sentence may carry.

For example, when your wife asks “ごはんとパン、どちらがいい？” (Would you like rice or bread?), you could answer “ごはんがいい” or “ごはんでいい.”
Both can be translated as “Rice is good.” But when you say “ごはんでいい,” you may sound like you’re NOT quite happy about the choices but you chose rice. On the other hand, when you say “ごはんがいい,” you sound you’re making the choice happily. Does this make sense?

So, when you say “これでいい,” you may sound like you’re not perfectly happy about the "thing" depending on the situation.
When the sentence is used as a question, you don’t need to worry about the nuance.
You’re right, Metaler-san. The particles in Japanese can be so confusing, but they are interesting!

Masako
Answered 5 years ago