One area of difficulty for students when they reach more advanced levels can be non-standard plurals, which in normal situations are wrong, but which in some circumstances seem to be acceptable. I’m thinking specifically here about people versus persons. Students will have learnt early on that if there’s more than one person, they need to use people, but then they might see a sign like this, and understandably feel confused:
Persons found to be in contravention of these regulations are liable to prosecution.
The key is the context – persons is the correct form to use in formal or official situations, like legal documents and public notices. The Get it Right box for person in the Macmillan Dictionary tells you more.