Yes, there are weak and strong forms in Dutch, just like in English.
Words that are pronounced with a weak form are pronounced with a schwa (the sound in een, en de...) (many unstressed syllables are actually pronounced with a schwa, as in English, but I don't want to drive you insane).
In the case of een, the difference in pronunciation is in order to be able to distinguish the indefinite article een (a/an) from number één (one).
In other cases, the use of strong/weak forms has to do with stresss placement (emphasis).
The nice thing about Dutch (when compared to English) is that the use of weak/strong forms is mostly signalled through spelling (ee/éé, e/ij, respectively, for instance).
Hope this helps and that I was clear enough.