Once you get in the habit of smoking, it's hard to stop.
Once you start smoking cigarettes regularly, it's hard to give them up.
ーOnce you get in the habit of smoking, it's hard to stop.
「一度タバコを吸うのが習慣になると、やめるのが大変だよ」
once で「一旦〜すると」
to get in the habit of ... で「…する習慣が身につく・〜の癖がつく」
hard to stop で「やめるのが難しい」
ーOnce you start smoking cigarettes regularly, it's hard to give them up.
「一度タバコを吸い始めると、やめるのが難しいよ」
regularly で「定期的に・日頃から・欠かさず」
to give up で「やめる」
ご参考まで!