-을/-를 is the object marker. It is placed after the object in a sentence. Sometimes, it is omitted but it is better for the object marker to be present.
In certain situations, the meaning of a sentence is often being understood even with the absence of the object marker:
-공부(를) 해요. Study.
-전화(를) 해요. Make a phone call.
-빨래(를) 해요. Do the laundry.
Other sentences using -을/-를:
-밥을 먹어요. Eat rice.
-옷을 사요. Buy clothes.
-친구를 만나요. Meet a friend.
It is also the same for 안/못:
-공부(를) 안/못 해요. Doesn't/can't study.
-전화(를) 안/못 해요. Doesn't/can't make a phone call.
-빨래(를) 안/못 해요. Doesn't/can't do the laundry.
-밥을 안/못 먹어요. Doesn't/can't eat rice.
-옷을 안/못 사요. Doesn't/can't buy clothes.
-친구를 안/못 만나요. Doesn't/can't meet a friend.