“遗憾”的形容词

应该有s，形容词用regretful怎么样？
或者换一个句型也可以:It's a pity that.....
What a pity it is that.....

因为 make 是I的动作 所以不是单三形式，不加s

a pity 是遗憾的意思 但是regretful才是遗憾的形容词

所以应该是：
What make I feel the most regretful is that I haven't know you earlier.

What makes me regretful/pity most is that I haven't know you earlier.
动词后用宾格

我觉得这样说更好些：
The most regrettable thing is that I haven't know you earlier。