Hello.. I was doing a grammar exercise and made some mistakes..can you tell me somehow why the correct ones were correct ?

(
1. This is generally considered to be the
___ guide to Australia ( DEFINE).
I used definited but definitive was right..
2. Greg's ____ to say the least , so i wouldn't ask him to choose a hotel. (DECIDE)
I wrote decided but indecisive was right
3. He was found not guilty by reason of ____and so was sent to a mental institution rather than prison (SANE)
I wrote sanity but insanily was right .. Just why??? The sentence means that someone was insane and thats why he was NOT guilty by reason of sanity... And he WAS GUILTY by the reason on insanity ...
Isn't it right ?:((((
Somebody help ???