* She had a boastful attitude, always bragging about her accomplishments. (This sentence describes someone who is generally boastful.)
* The salesman's boastful claims about the product were quickly debunked. (This sentence describes a specific boastful action.)
* His boastful tone made everyone uncomfortable. (This sentence focuses on the way a boastful attitude is conveyed.)
* It's important to avoid being boastful, even when you have something to be proud of. (This sentence provides a general piece of advice.)
You can also use "boastful" as a noun:
* His boastful was met with skepticism. (This sentence uses "boastful" to refer to the act of boasting itself.)
The best way to use "boastful" in a sentence depends on the context and what you want to emphasize.