我理解成呢兩個意思啱唔啱？

could - modal verb

1. past ability
~ used as the past tense of 'can' to say what someone was able to do or was allowed to do in the past:

You could leave your bag at the front counter while shopping in this shop.

2. possibility
could + have
~ used when saying that it was possible for somebody to do something in the past but they did not try:

You could have left your bag in the shop but decided to take it away with you.

THE ANSWER is:
a. 你本來能夠將袋留在店內，但你沒這樣做。

A) correct eg:-Formally, They could have let me know they were going to take away my bag due to a certain reason.
eg:-They could have done with a left-over for the bag in the shop(hence for me to act )
eg:-Used to emphasize how strongly I want to express my feeling(hence they have done wrong for not leaving the bag for my usage !)

B)Used to show that something is or might be possible;
eg:-You might leave it there,if you think it is too" heavy".
eg:-Don't worry-----the bag could have just left there for me to just "pick-up".
eg:-You couldn't have left it in the shop, could you?.."Oh, I couldn't,thank you" But I might.(due to heaviness.)

A正確，B用MAY或MIGHT。