Hi,
a) It became an unforgettable day for Susan.
(b) When Susan was choosing the books in the book store, suddenly she heard a commotion.
(c) She realized the girl was lying on the floor.
(d) When she arrived, there were many people gathered at the spot.
Yes, these are OK. Well done!![Smile [:)]](/emoticons/emotion-1.gif)
Clive