Ex.1
1) employee
2) unexplained
3) went missing
4) residents
5) go through
6) mysterious
7) go very well
8) going for
9) refugees
10) staff
Ex.2
1) couldn't have
2) might have
3) who
4) which
5) couldn't have
6) must have
Ex.3
1) has → have
2) might like → might have liked
3) which → who
4) She's the runner who won the gold medal.
5) be → have
Ex.4
1) A: can't ; B: might
2) A: really ; B: worries
3) A: phenomenal ; B: incredible
4) A: quite a ; B: amazing