The verb study describes an activity, the activity that you undertake when you want to learn about something.
In contrast, the verb learn focuses on the moment when something has become part of your knowledge. The verb learn also contains a sense of completion and permanency; usually, when you have learned something, you know everything about it and you don’t easily forget it.
The sentence below sounds strange because the verb learn gives a sense of completion. If you have truly learned something, you are normally able to use that knowledge.
Strange: I learned English in school, but I can’t speak it.
It’s possible to study something without learning it, however, so the sentences below sound fine.
Natural: I studied English for two years, but I can’t speak it very well. Natural: I studied very hard, but I didn’t learn much.