Well, There might be a few good or less good reasons for not doing all the exercises, FE and oefenbrief 1
1. Your English is quite good so you don't need the practise- no problem but do the FE to be sure your knowledge is up to par
2. You didn't feel like it..... No problem, however check if you understand the grammar and know the words in FE otherwise you might get into trouble
Handing in the OEFENBRIEF is obligatory (verplicht)