Examples

I can't say more in case I let the cat out of the bag.

我不能再说什么了，免得泄密。

I can't say I'm sorry

我并不感到内疚

I can't tell you {or} say off-hand

我不能即刻告诉你

Don't be wishy-washy; you can't go wrong by listening to what I say.

别犹豫了，听我的话没错。

now, I can't really say

得蒙先生指教，如拨云见日，感激不尽。

You have dispelled the clouds to let me see the sun, sir. I can't say how grateful I am for your advice.

我不了解细情，所以不好说。

I can't say for sure because I don't know the details.

我说不好她肯不肯来我们这儿工作。

I can't say whether she would like to come and work here or not.

我说不上他来不来。

I can't say whether he is coming or not.

众人笑我太痴癫，我笑他人看不穿。

Others laugh at me and say I've gone mad; I laugh at them for they can't understand.