"I don't understand this sentence."

Translation: 我不明白这个句子。

[Patrick_Dark]

We learned to use "懂" in a previous lesson, so allowing "不懂" here would seem to make sense.

[KX3.]

Yes, but that is more colloquial.

[baker3060]

Is 个 really the best measure word to use for 句子?

[KX3.]

I think so.