ok,for the first question...when you can use mesti,harus and kena...mesti and harus is a bit formal word but still have the same meaning...the difference between mesti and harus is that 'mesti' is something like a force word.'kena' is quite informal word which we commonly use to talk with our friends.. e.g: saya mesti pergi kerja = i must go to work....you are forced to go to work saya harus pergi kerja = i should go to work...you are willing to go to work saya kena pergi kerja = i have to go now... question 2: 'saya sakit, saya tak boleh pergi kerja hari ini' or 'saya sakit, saya tak dapat pergi kerja hari ini'. in terms of those sentences,it has the same meaning...boleh and dapat is actually 2 different words.it depends on how and where you use it.. i hope i can clear up this matter for you...maybe it is a bit difficult to understand...i'm sorry...