簡單問題........

Firstly, there is a grammar problem in the question already.
" What are you doing this evening? " is wrong.

The correct ones should be:
1. " What are you doing? " (Means you are doing something right now, but this is not the meaning you want for the case.)
2. " What are you going to do this evening? " (Means you have planned to do something this evening.)
3. " What will you do this evening?" (Means either you may have a plan, or you have no plan at all.)

What are you doing this evening?
你今晚打算做甚麽?

點解要用continue tense唔用future tense?

因為這是肯定詞.
例如:
Tomorrow is Sunday. 你不可說做 Tomorrow will be Sunday. 因如今天是Saturday, 明天則肯定是Sunday.

用continue tense係因為問你有咩plan 左係evening..
有一個計劃去做

因為係現在問,不是將來問,so用continue tense
係唔係應該係What are you want to do this evening?