2-He hasn't done anything so that we can arrest him.
This one is wrong. "So that" introduces the effect from he hasn't done anything!!! How can you arrest him? So this one should be:
He hasn't done anything so that we can't arrest him.
Thanks Tombraiders and TDOL.
Tombraiders' sentence got me a bit confused at the start, but then I got it. It really helps. TDOL's reply is as cool as ever. Just imagine what he will do if he is in power!
I'd like to ask another question.
Which of these is correct:
3-"They have done something so that my phone does not work."
4-They have done something for my phone not to work.