Yap.Town

を見分けるのは

distinguishing ... is

→ を
phrase Rare #34,884
distinguishing ... is Used in the pattern 「AとBを見分けるのは〜」 to say that telling A and B apart is easy, difficult, or another quality. The のは nominalizes the verb phrase and marks it as the topic.
JA: 双子を見分けるのは難しい。
EN: It’s difficult to tell the twins apart.

彼と彼の兄を見分けるのは簡単だ。

It's easy to tell him and his brother apart.

君と君の弟を見分けるのは難しい。

It's hard to tell you and your younger brother apart.

本物と偽物を見分けるのは困難だ。

It’s difficult to distinguish between the real thing and a fake.

日本人と中国人を見分けるのはいつもやさしいとは限らない。

It isn't always easy to tell Japanese people from Chinese people.