The combination “是-不-是” literally translates to “is-not-is?”. This structure forms what linguists call an “A-not-A question.” By presenting both the affirmative and negative possibilities, you are asking for confirmation about a statement you already suspect is true.
While “是不是” is a grammatical tool, its frequent use reflects a subtle aspect of Chinese communication culture: a preference for indirectness and consensus-building. Asking a direct question with `吗` (e.g., `你是美国人吗？` - “Are you American?”) is perfectly fine, but can sometimes feel a bit blunt or clinical, like filling out a form. Using `是不是` (e.g., `你是不是美国人？` - “You're American, right?”) softens the question. It implies the speaker has already made an observation and is simply looking for confirmation. This creates a smoother, more collaborative conversational flow. It's less like an interrogation and more like a shared discovery. Compared to the variety of English tag questions (“isn't it?”, “aren't you?”, “don't they?”), the single, versatile “是不是” is a powerful and efficient tool for achieving this conversational harmony.
“是不是” is incredibly common in all forms of modern Mandarin. Its placement in a sentence can slightly change its nuance.