翠星石 (nprofile…vqps) you need a dictionary that can understand inflected words. in this case, わかんのか?⇒わからないのか?⇒わからない⇒分かる
Rikaitan has a deinflector built-in. jisho.org can deinflect words. goldendict-ng can deinflect words if used together with gd-tools.