I'm sick, but I must (do) work.

it's seven day with cough and cold I've had a cough and a cold for seven days (Or for a more literally translation you could say: It's my 7th day with a cough and a cold) but I have my activities and i must to work activities and work to do.

I would like to stay at home and to sleep!!!

Also: Remember that "I" (by itself) is always capitalized, unlike "io"!

I am sick,  but I must go to work

Its the seven day with this cough and cold but I have my activities and I must go to work . I would like to stay home and to sleep !!!