هُمْ and هُمُ are one word but we say هُمُ when the next word starts with a letter with sukoun ex: هُمُ الْذين in this example it is followed by letter لْ with sukoun ( note : in the word الْذين we don't pronounce letter ( ا ) so we pronounce this example in this way هُمُ لْذين

I'm not sure but I believe what you mean is when you say: where are they? And when say They are very poor people. I don't see any difference, in Arabic they is they. wherever you put it, it has the same sound, same meaning. There's one way I can think of, and you can hear that mostly in poems,: هم الرسل الكرام فسر .. على منهاجهم تربح When you hear it, it's sounds like هموا but it's actually هم sometimes in Arabic the sound track you into to believe it's a different word but it's just the same, bottom line, my advice to anyone wants to learn Arabic, don't put too much pressure on yourself, just let it happen, just like God said " لا تسالوا عن اشياء ان تبدا لكم تسؤكم" again it's my personal oopinion