when inuyasha is a half demon he can change into a full demon when his life is in risk atau before hes gonna die. here is my question:when inuyasha is in his human form and he gets attacked,why cant he just turn into a full demon in his human form

because no yokai blood flows though his vein anymore and if there are no yokai blood, then it cannot increase and manipulate his human and hanyo blood.

he loses his demonic powers when he turns into his human form that's why.

He loses his powers at the night of the new moon and transforms back when the sun rises

thats 'cause he loses his yokai powers on the night of the new moon

Because at this time, he is a human, and humans cant turn into demons

First of all it is inaccurate to say he can change into a full demon. In reality his demonic blood takes control giving him something like a rush of adrenaline. In his human form the demonic blood is deactivated completely for a period of time although I can speculate it can be awaken even then.