The difference between relative clauses and embedded question complement wh-clauses
can be very tricky to identify.
- all of the sentences presented contain embedded question complement wh-clauses,
- none of the sentences presented contain restrictive relative wh-clauses.
Relatives and questions use almost the same set of wh-words.
There are some differences, though.
For one thing, whether, how, and what can never be used as relative pronouns:
- **the answer whether he was coming*
- **the way how he walks*
- **the car what he drives*
why is severely restricted in what nouns it may modify as a relative pronoun; basically only reason:
- the reason why he came, but not **the cause/purpose/intent(ion) why he came*
These restrictions on wh-words contribute to the frequency of that in relative clauses,
since that can be used instead of any wh-word, and is not restricted like wh-words are.
Complements, on the other hand, are clauses used as nouns,
while relatives are clauses used as adjectives, to modify nouns.
I.e, complement clauses can be the subject or the object of
clauses that have the appropriate (complement-taking) predicates.
The first, second, and fourth original sentences contain direct object complement clauses,
governed by the respective transitive predicates remember, remember, and discover.
The third original sentence contains a subject complement clause,
governed by the complex predicate remains a mystery.
(This predicate, btw, has already been done some things to;
for more details than you need on that subject, see here.)
That's enough to be going on with for a while.