*Today, I went to the ferret shop to wash my pet. Ferrets smell so bad, so I have to be careful.* - In English, we always put a space after a comma. - "I must care" is incorrect here because "to care" alone means to think something is really important, to like something a lot, or to be very interested in something. For example, "I care about my ferret" means the ferret is important to me, I like him/her a lot. I think you were looking for "to be careful" or "to take care", meaning to pay attention to something, to avoid something bad happening. You have to be careful to wash your ferret, otherwise he/she will smell bad.