An adverb meaning 'how' or 'how much' — used to ask about or emphasize degree or extent.
どんなに嬉しかったか、今でも忘れられない。
I still can't forget how happy I was.
どんなに遠くても君に会いに行くよ。
No matter how far, I'll come to see you.
どんなに高くても欲しいものだ。
No matter how expensive it is, it's something I want.
Used in concessive phrases like 'どんなに ... ても/でも' to mean 'no matter how (much)...' — indicating that the outcome doesn't change despite the degree.
どんなに練習しても上手くならないこともある。
No matter how much you practice, there are things that won't improve.
どんなに反対しても、彼は行くつもりだ。
No matter how much we object, he's going to go.
どんなに疲れても約束は守るべきだ。
No matter how tired you are, you should keep your promises.