Adding "ma" to a statement turns it into a yes/no question. Without it, the sentence is complete and correct, just not a question. "你是中国人 "means "you are Chinese." "你是中国人吗” is a question. Are you Chinese? Or not? As far as I know, this is all the particle "吗” does. Sometimes the question is rhetorical, but the grammar doesn't really change. If there are other uses, they're advanced enough that you don't need to worry about them yet. "Ne" is different. It's used when the question is implied rather than directly stated. For example: "我有一个妹妹。你呢？” I have a little sister. You? Here, I'm asking if you have siblings or not. But rather than asking the comparatively long question "你有兄弟姐妹吗？” I can just affix "呢” the the end of the 2nd person pronoun. The question is understood from context. ”呢” has other uses too, but I wouldn't worry about them for now.

吗和呢都是语气助词，吗表示提问：你好吗？她很漂亮吗？你吃饭了吗？ 呢多用于表选择或疑问：你是吃饭还是吃面呢？她是中国人还是日本人呢？我有一个弟弟，你呢？我喜欢养狗，你呢？ 呢也可用于感叹句表肯定：我好着呢！