"I used to go to jail."
Translation:Tôi đã từng bóc lịch.
Not sure if this is what the Vietnamese sentence is trying to go for, but in English, the "used to" construct usually implies a habitual action.
So, I would assume either this person used to work in the prison as a guard or a warden, OR this person kept getting arrested, put in jail, paroled/released and then started this vicious cycle all over again.
Also 'go to' when used like this implies going voluntarily (people are usually 'sent to' jail; you only usually say 'go to jail' when talking about sentence length - 'he went to jail for 5 years')
Plus, 'used to' implies a change in the action - you don't do it anymore.
The only way I can think this sentence would sound natural - 'I used to go to jail' - is if 'jail' was the name of a trendy club or something: 'I used to go to Jail but then they changed the music; now I go to Heaven.'
As often seems to be the case, "used to" doesn't seem to work as a translation of "đã từng" when "once" makes perfect sense. "I once went to (spent time in) jail" seems closer to the meaning. Another example is "America once (đã từng) declared war on Japan" which shows up elsewhere in this course.
Have you ever seen a block calendar? They were quite common when I grew up in Vietnam many years ago. A block has 365 pages, each page is for one day. So as a day goes by, you peel the page of that day off. This idiom is used to mean 'being in jail' where one passes his time by peeling one page off the block calendar each day!