The key answer of exercise 6

a)

1. whatever
2. whoever
3. what
4. who
5. how
6. when

b)

1. I practise speaking in English with whoever I can find.
2. Good subtitled films are exactly what I need to improve my listening.
3. ✓
4. What we're studying today is the same in my language.
5. A good app for vocabulary learning is just what I've been looking for.
6. Little and often. That's just how I like to learn.
7. I revise vocabulary whenever I'm on public transport.
8. Whatever I do, I don't seem to be able to manage phone calls in English.
9. ✓
10. I keep a record on my tablet of what we've studied. [It would also be possible to use a normal relative clause here ('I keep a record on my tablet of everything (that) we've studied.')]