Original sentence: What everyone thinks is a narwhal’s horn is actually its tooth!
Is it correct? Yes — the sentence is grammatically okay. It tells an idea clearly: people think narwhals have a horn, but that horn is actually a tooth.
Small style suggestion: Instead of "everyone," you can say "many people" because not every single person thinks that. For example:
- What many people think is a narwhal’s horn is actually its tooth.
- The thing many people call a narwhal’s horn is actually a tooth.
- A simpler sentence: A narwhal’s horn is actually a tooth.
Two small grammar tips for you:
- Use narwhal’s with an apostrophe to show the tooth belongs to the narwhal. That is correct.
- Use its (no apostrophe) when you mean "belonging to it." Do not write it's here — that's short for "it is." For example: "It is cold" = "It's cold." But "the dog wags its tail" shows the tail belongs to the dog.
Nice sentence — great job! Keep writing and asking questions.