Thank Uhrwerk & MrCode!
As you two said,Yes. I've tried some commands just like phent_addvelcentral and phent_addcentralforce. But the entity never walked forward but roll forward or aside. I am really at a loss. The code I tried is like:


VECTOR guard_speed;
vec_set(guard_speed.x, nullvector);
guard_speed.x = 25 * (key_a - key_d);
guard_speed.y = 25 * (key_w - key_s);
phent_addvelcentral(guard, guard_speed); //roll
// phent_addcentralforce(guard, vector(50,0,0));//still roll

In fact I would appreciate you so much if there comes some code to make it from you.