Previous means “immediately before,” and former means “at any time in the past.”
▶️ Your former employer means any employer you have had in your past, at any
time.
▶️ Your previous employer typically means the most recent employer you had
in your past.
▶️ A former president is any of the many people who have held the position of
president in the past.
▶️ The previous president is the most recent president before the current one.
We usually use ex- with romantic partners from the past: ex-husband, ex-wife, ex-
boyfriend, ex-girlfriend. Another common term is ex-convict or ex-con: someone
who has been convicted of a crime and spent time in prison, but has now been
released.
Although it’s possible to use ex- with other roles, it’s not very common. “Ex-” is
rather informal. However, it is sometimes used in newspaper headlines to save
space (Ex-leader of an organization, ex-foreign minister of a country, etc.)