My favorite instrument, the trumpet, has a long history.
Here's why:
* "My favorite instrument" is a phrase acting as the subject of the sentence.
* "the trumpet" is an appositive, meaning it renames or clarifies the subject ("my favorite instrument"). Appositives are usually set off by commas.
* "has a long history" is the predicate of the sentence.
Let me know if you have any more punctuation questions!