"I forgot just one time."
Translation:저는 딱 한 번만 잊어버렸어요.
While the supplied translation is correct, it seems that there are various ways to say this, especially given that the first two words in the correct response can be seen as implied, in the Korean language. (Imho, as a native speaker who is trying to use this app to improve her written skills in the language...)
- 잊다 = "to forget"
- 잊어버리다 = "to (completely) forget"
For the most part these two verbs are synonymous. The latter verb carries a connotation of more time passing or forgetting to a larger extent.
Note: 버리다 is a common auxiliary verb that you attach to other verbs to indicate "completeness" of the action.