I've never heard someone say "ii yo". Is "ii yo" acceptable? And "ii zo"? In particular, in casual masculine speech.
In English, it would be similar to casually saying "yeah, sure!".
- Can I borrow your pen?
- Yeah, sure!
いいぞ, is also correct but it's less standard.
I never heard people actually use that in my experience. But people talk in all kinds of weird ways. Imagine someone saying the following in English:
It's not standard but some people actually talk like that so culturally speaking, it's not wrong.
いいよ (ii yo) is a casual way to say "okay", "no problem", "I'll do it", etc.
- It is not acceptable in formal/business settings. Don't use it.
- It is commonly and actively used among friends, regardless of sex.
- It is not common in some dialects where other expressions are used by default. Kansai-ben speakers would use ええよ or ええで.
いいぞ can be heard when cheering sport players (sounds like "way to go!"), but otherwise uncommon in the real-world spoken Japanese. You can say this to someone who is really inferior to or close to you (for instance your children), but it tends to sound "bossy".