I decided to keep this cat inside rather than let him/her go outside.
I decided not to let this cat wander around outside. I'm going to keep him inside all the time.
ーI decided to keep this cat inside rather than let him/her go outside.
「この猫は外に出さないで、室内で飼うことに決めた」
to decide で「決める」
to keep this cat inside で「この猫を室内で飼う」
ーI decided not to let this cat wander around outside. I'm going to keep him inside all the time.
「この猫は外に出さないことにした。常に室内で飼うよ」
to wander around outside で「外をウロウロする・歩き回る」
all the time で「いつも・ずっと」
ご参考まで！
カナダ 
