Can you use "hedefim" instead of "amacım?"
Of course :)
Why does "amaç" becomes "amacım" when conjugated?
Read this: https://www.duolingo.com/comment/9040173
Many words that end in a t, k, p, or ç undergo consonant mutation :)
What is the difference between 'hadef' and 'amaç'?
*Edit: I have already found an answer! (Amaç: aim, purpose, objective; Hadef: goal, tarject, objective) (orde90 commented that)
kullanmak is not accepted. Is it wrong?
Yes it is wrong. "Kullanmak" = "use"