- I could work on Saturdays.
Answer: It's possible for me to work on Saturdays. - I must work on Saturdays.
Answer: It's absolutely necessary for me to work on Saturdays. - I need to work on Saturdays.
Answer: It's important for me to work on Saturdays. - I have to work on Saturdays.
Answer: It's absolutely necessary for me to work on Saturdays and someone forces him to do so. - I mustn't work on Saturdays.
Answer: It's forbidden for me to work on Saturdays. - I couldn't work on Saturdays.
Answer: I'm unable to work on Saturdays. - I don't have to work on Saturdays.
Answer: It isn't necessary for me to work on Saturdays. - I shouldn't work on Saturdays.
Answer: It isn't a good idea for me to work on Saturdays. - I should work on Saturdays.
Answer: It's a good idea for me to work on Saturdays. - I might work on Saturdays.
Answer: It's a possibility that I might work on Saturdays.
Did you have a hard time understanding? Here is a quick guide to basic modal verb usage.

