افضل طريقة لكتابة هاتان الجملتين هو ااستخدام الجملة الفعلية لانها تدل علي التجدد.
١- لا يزال بإمكانه العمل اليوم.
٢- لم يعد بإمكانه العمل اليوم.
The word (لا يزال) means that the day is not over yet and he still can work today. In the second example, the word (لم يعد) is a negation which means that the man cannot work today cause it's over.
The verb ( يمكن) expresses possibility or ability and it equals the verb (can) in English. It does not express an obligation.
يمكن =can.
I hope that will help you to differentiate between them. Wish you best of luck. :)