verb base + 는 거야 = verb base + 아/어 verb base + 는 거예요 = verb base + 아/어요 translations are the same. Nuances can be slightly different in Korean, but the general meaning is usually the same.

뭐 하는 거야? vs. 뭐 해요? Their translation to English would be the same: What are you doing? But ~는 거야 kinda more focuses more on the action itself, and in everyday speech it's used less than simple conjugation (해요). ~는 거야 feels like it's more serious. 나를 사랑해? - Do you love me? 날 사랑하는 거야? - Do you love me?(are you really serious about it?) 선물로 받았어. - It was a present. 선물로 받은 거야. - It's someting I received as a present.

V + -는 거야 used for explaining the action about which the listener has some information. V + -고 있어/다 used for explaining the action about which the listener has no information. For example, Q: 지금 뭐 하는 거야? : What is that you are doing now? (The speaker saw some actions or heard some noises.) A: 책 읽는 거야. : What I'm doing now is reading a book. Q: 지금 뭐 하고 있어?/지금 뭐 해? : What are doing now? (The speaker has no information) A: 책 읽고 있어/ 책 읽어. : I'm reading a book.