How do I structure my database so that two tables that constitute the same "element" link to another?

Question

I read up on database structuring and normalization and decided to remodel the database behind my learning thingie to reduce redundancy.

I have different types of entries that can be learned. Gap texts/cloze tests (one text, many gaps) and simple known-unknown (one question, one answer) types.

Now I'm in a bit of a pickle:

• gaps need exactly the same columns in the user table as question-answer types
• but they need less columns than question-answer types (all that info is in the clozetests table)
• I'm wishing for a "magic" foreign key that can point both to the gap and the terms table. Of course their ids would overlap though. I don't like having both a term_id and gap_id in the user_terms, that seems unelegant (but is the most elegant I can come up with after googling for a while, not knowing what name this pickle goes by).
• I don't want a user_gaps analogue to user_terms, because then I'd be in the same pickle when it comes to the table user_terms_answers.

I put up this cardboard cutout collage of my schema. I didn't remove the stuff that isn't relevant for this question, but I can do that if anyone's confusion can be remedied like that. I think it looks super tidy already. Tidier than my mental concept of this at least.
Did I say any help would be greatly appreciated? Answerers might find themselves adulated for their wisdom.

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Background story if you care, it's not really relevant to the question.
Before remodeling I had them all in one table (because I added the gap texts in a hurry), so that the gap texts were "normal" items without answers, while the gaps where items without questions. The application linked them together.

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Edit

I added an answer after SO coughed up some helpful posts. I'm not yet 100% satisfied. I try to write views for common queries to this set up now and again I feel like I'll have to pull application logic for something that is database turf.

Answer 1

As mentioned in the comment, it is hard to answer without knowing the whole story. So, here is a story and a model to match. See if you can adapt this to you example.

School of (foreign) languages offers exams for several levels of language proficiency. The school maintains many pre-made tests for each level of each language (LangLevelTestNo).

Each test contains several (many) questions. Each question can be simple or of the close-text-type. Correct answers are stored for each simple question. Correct terms are stored for each gap of each close-text question.

Student can take an exam for a language level and is presented with one of the pre-made tests. For each student exam, the exam form is maintained which stores students answers for each question of the exam. Like a question, an answer may be of a simple of of a close-text-type.