Are these sentences correct?
b) He told me about many cities he had visited during his travelling.
c) I couldn’t answer teacher`s questions because I hadn’t revised anything before the exam.
d) When we came to the theater,the play had already started.
e) I went to watch TV after I had written a letter to my friend.
f) She couldn’t find a book I had lent her.
g) She told me that she had worked in France and Germany.
h) When I came to the car parking I realized that I had lost the keys.
i) It was a firm I had never heard before,
j) My headache disappeared after I had taken medicine.
Are these sentences correct? I doubt about using Past perfect correctly here.