I was told how an older non-native lady would say "it's okay!" and that it was perceived to be just like that person and cute many because it was grammatically incorrect. She would say, いいだよ
Now I didn't know that rule but I was sure I had heard いいだよ before. It is such a simple statement but even these things trip me up.
So I'm guessing the rule is like
だ after な-adjectives, nouns
No だ after い-adjectives
Can I get something definitive on this please? What about adverbs?