Hang on, isn't 'er' plus past participle the perfect? I put 'have been forced' and it said it should be 'are forced'. But 'de er kommet' is they have come. So why is 'de er tvunget' not they have been forced?
they are forced = de er tvunget
they have been forced = de er blevet tvunget
It works pretty much the same way as in English.