In projectile motion...(M2)

Why is the velocity of a particle projected from ground, for example, have a velocity of 0 at the max height of the parabola it makes?

The vertical velocity would be 0 but there would still be horizontal velocity.

For projectile questions it's useful to consider horizontal/vertical motion completely separately.

So for the vertical motion : You have a particle projected upwards with some positive veloctiy. g = -9.8 so the velocity decreases until it reaches 0 at the max height. Then the velocity becomes negative and gets more negative until it reaches the ground.

Have you done AS physics?
When you first project the particle form the ground it has maximum vertical and horizontal velcoity(if you threw it with a horizontal velocity as well rather then just staright up), because the acceleration due to gravity is downwards, as the ball goes UP the accleration tries to pull it back down so it reduces and reduces and when its at the top it reaches a maximum height and the vertical velocity is zero,however the horizontal vel. never changes
it then starts to accelerate downwards again in the opposite direction and go back to the ground

Put in more hours, get a perosnal tutor if you have to and study hard and im sure you can bump up your grade, do papers from different boards as well.
http://www.physicsandmathstutor.com
Also check out isaacphysics