I planted sweet william from seed last year, thinking it was an annual. I had lovely, lush plants that never flowered. The plants overwintered, and I am wondering if I should just let them be, or do I trim them back and will they bloom this summer?
You don't indicate where you are located which helps answer questions. In my area of the country Sweet Williams are perennials and grow wild. We transplanted some of the wild ones a few years ago and they keep coming back. Pretty little things and a nice addition to the yard. Ours will bloom as the temperature rises later this spring.
They are a perennial where I live also. I see no reason why they should not bloom for you this year. They like to be split every couple of years also, and don't be afraid to give them a haircut when the blooms get sporadic. This will encourage more blooms throughout the growing season.