if you know how to create a variable in trigger editor, you have noticed that editor allows you to define variables of type quest and variables of type quest requirement. so once you create a quest, you put "set Q1 = (Last created quest)" as a next action and you can acces it later - for example, player killes some mini-boss and you do "Mark Q1 as completed" or a part of it instead of the whole quest...
the best way to learn that if you are still having problems is from blizzard campaign maps. they are all unlocked ad stored within mpq files in game directory. just play some campaign level ingame, see how thing work, then extract the map from mpq file and open it in world editor to look inside.