が理解できない

can't understand

→ が
phrase Common #6,159
can't understand Used after the thing that cannot be understood. 理解できない is the negative form of 理解できる, meaning “can understand.”
JA: この説明が理解できない。
EN: I can't understand this explanation.

用事やり方が理解できない

I don't understand how to handle the task.

— ダンガル きっと、つよくなる (2016)

しかし私たちは彼の言うことが理解できない

However, we can't understand what he's saying.

私にはどうしてもこの聖書からの一節が理解できない

No matter how hard I try, I just can't understand this passage from the Bible.

僕は彼女の言ったことが理解できない

I don't understand what she said.

数学が理解できない

I don't understand math.

私は彼の要求しているものが理解できない

I don't understand what he's asking for.

私には問題の微妙なが理解できない

I don't understand the subtle points of the problem.

私にはその問題が理解できない

I don't understand that problem.

彼には友情というものよさが理解できない

He can't understand the value of friendship.

私は君の言う事が理解できない

I don't understand what you're saying.

やり方が理解できない

I don't understand how to do it.