The thirty-first runner crossed the finish line just seconds after the leader.

After dividing the cake into thirty-one equal parts, he handed me the thirty-first.

The forty-first contestant smiled nervously as her name was announced.

After dividing the cake into forty-one equal slices, he handed her the forty-first with a smile.

Despite the risks, the first movers in the market secured a lasting competitive edge.

I check my emails first thing.

The first thing to understand about chess strategy is controlling the center.

Engineers often return to first principles when designing a truly reliable system.

When redesigning the product, we returned to first principles to avoid repeating old mistakes.

When teaching beginners, I often explain the first conditional with real-life examples to show how an if-clause in the present tense and a main clause with will + infinitive express possible future events.