Unexplained grammatical exceptions
 

Do you know any formal constructions of grammar in which a lot of unexplained exceptions to a given grammatical rule exist, e. g. irregular verbs or the plural?

So far I have identified the following areas:

1. non-passivizable transitive verbs
2. irregular verbs
3. irregular adverbs
4. tenses
5. plural

Do you know any other formal grammatical areas that contain unexplained morphological, syntactical or phonotactical exceptions? Usually foreign learner of English find these constructions particularly difficult to master.

Thanks

Re: Unexplained grammatical exceptions
 

*push*