It' careless for you to make so many mistakes改个错

It' careless for you to make so many mistakes改个错
It' careless for you to make so many mistakes
改个错

It' careless for you to make so many mistakes改个错
It' careless of you to make so many mistakes
of是用来形容人的
而for用来形容物 careless自然是形容人的

for->of
for是表示对于某人来说，那个物品的性质。
of是某个人的性质。
这里careless是you的性质，所以用of.

for 变 of ,careless(粗心）形容的是人。

it is