Is there an idiom for "something that occurs for zero or infinite times"?
I'm looking for a word or phrase for the warning "If you do it for once, you will do it next time, and until infinity", usually with a negative connotation (like warning someone against taking drugs or other harmful and addictive things).
The Chinese phrase that conveys the meaning perfectly is 有了第一次就会有第二次, which literally translates to "If there's the first time, there will be a second time".
The phrase should probably fit as a noun in terms of "part of speech".
One idiom is slippery slope, meaning that one step will inevitably take you much further, and you cannot stop. Farlex has
A dangerous and irreversible course
the slippery slope from narcotics to prison
and Lexico has
A course of action likely to lead to something bad or disastrous
he is on the slippery slope towards a life of crime
There exist a family of idiomatic forms which allows one to specify the particular situation they have in mind; it is "once a (something), always a (something)".
(Free Dictionary) once a (something), always a (something)
proverb A person's innate or fundamental nature is not something they are willing or able to change. ♦ He says he's trying to leave his life of crime behind, but once a criminal, always a criminal.
♦ Even years after leaving the stage, she still can't resist performing—once an actor, always an actor.
In using it, you can't generalize, and you have to find the word that fits the given situation.
- Once a crook, always a crook. (ref.)
- Once a liar, always a liar. (ref.)
- Once a leader, always a leader. (ref.)
- Once a teacher, always a teacher. (ref.)