Is this sentence correct?

[RoseSpring]

Is this sentence correct?

He started saying things about me which I didn't understand.

 

[2010]

Is this sentence correct?

He started saying things about me, which I didn't understand.

===Not a teacher===

Use a "comma" before "which".

Your sentence sounds contradicting.

The sentence says that you knew the other person was taking about you, which means, you did understand the topic, but it was probably not true.