A level mechanics help

Help I don’t get why I couldn’t just do

This…
Because velocity (V) is 0 for the vertical
Because it hits ground



Also for other Qs if I take up to be positive will both distance and acceleration be minus

Answering your first question, that is actually a very common misconception.
A creative way to think about it is imagine you're the particle. Suvat models the movement of the particle under constant acceleration between two points. The particle itself does not know where the ground is so it wouldn't make sense that it naturally comes to rest as soon as it hits the ground.
Another way to think about it is that v=0 means the particle brings itself to rest before it hits the ground. Again this doesn't quite make sense as the particle is clearly moving with a certain amount of speed when its hitting the ground. Hence we say its "hit" the ground.
As for your last question, what you said is correct. Opposite directions have opposite signs. If you're taking upward movement as positive then anything downwards must be negative.

So how do I know when v is equal to zero