一緒に住んでいるけど付き合ってはいないの?ルームメイトだということだよね?
Hi everyone, I’ve encountered this example and I’m a bit confused about the 付き合ってはいないの? construction. Would it also be grammatically correct to omitt the は? And why is の used a question marker particle? I searched online and apparently this is a fair example, since は can be inserted as plain contrast marker and の can be used as a question marker particle, but I do not recall having seen this points discussed in the previous lesson, but I probably overlooked them, therefore I’m making this post to search for a confirmation.