"I miss him/her terribly."
Here's a breakdown:
* wo (我) means "I"
* xiang nian (想念) means "miss"
* ta (他/她) means "he/she"
* ke shi (可是) means "but"
* hen (很) means "very"
The sentence structure emphasizes the speaker's deep longing for the person. The use of "ke shi" suggests a feeling of sadness or longing that is difficult to bear.