Hoi Monika, great question! We use 'Wij' when there is emphasis: "Wij zijn verantwoordelijk voor dit probleem, jullie niet." Use 'We' when there is less emphasis on the word: "We gaan morgen naar Amsterdam." Often 'wij' is used the first time when introducing the persons in question. 'Wij' can also indicate a contrast with other persons: "Not they, but we have to solve this." (like the first example) However, the two are interchangeable and can both be used in most sentences. The same applies to: mij - me, jij - je and zij - ze