Each lesson assumes you have studied the prevous ones so it is strongly adviced to study them in the order they appear. Each of them has a reference to the relevant Grammar section. Spend as much time in each one of them as you feel it is necessary to understand and master its material. Do a revision each time you feel you have a gap in your knowledge or there is something that slips your memory.
At the end of each unit there is a revision test. It is advisable to succesfully take that test before proceeding to the next unit.
And remember to take as much time as you need in each lesson. It is you who sets the pace.
