"Why does your boyfriend only sing at night?"
Translation:Waarom zingt jouw vriendje alleen 's nachts?
No, it's only with the pronoun 'JE / JIJ'. 'Je zingt' (WITH 't' because 'je' comes before 'zingt') means 'you sing', but 'Zing je ?' (without 't' because 'je' comes behind 'zingt') means 'Do you sing ?'. (a question, it's true). BUT it's the same even if it is NOT a question, like when 'je' has to come behind the verb, for example 'Vanavond zing (without't') je ons een liedje.' ('Tonight, you'll sing a song to us'). But it is is ONLY with 'JE /JIJ', in the 2d person. Here, the subject is 'jouw vriendje', it's also 3d person, and 'zingt' CANNOT change. I hope it's clear.