がさっぱり分からない

have absolutely no idea

→ が
phrase Rare #36,764
have absolutely no idea Used with a negative form of 分かる to strongly emphasize that something is completely incomprehensible. 分かりません is the polite version.
JA: この問題の答えがさっぱり分からない。
EN: I have absolutely no idea what the answer to this problem is.

彼の考えがさっぱり分からない

I have no idea what he's thinking.

ドイツ語がさっぱり分からない

I don't understand German at all.

理由がさっぱり分からないんだよ

I haven't the slightest idea why.