It’s a one. When deciding whether to use “a” or “an”, you want to choose based on the sound the word begins with, not the actual letter it begins with. Since “one” starts with a “w” sound, “a one” is the right one.

It’s a one. When deciding whether to use a or an, you want to choose based on the sound the word begins with, not the actual letter it begins with. Since “one” starts with a “w” sound, “a one” is the right one.

The player wrote a one on his scorecard.

The player wrote an one on his scorecard.

Actually, many native speakers pronounce “one” and “won” in the same way.

They won a one-year contract.

More examples:

She rolled the die and got a one.

The lover wrote a one on her hand.

The number 51 is made up of a five and a one.

Follow the same strategy with other related phrases that include a compound adjective starting with “one”, such as “a one-day event”, “a one-day training”, “a one-way ticket”, “a one-way street”, “a one-off”, etc.

She bought a one-way ticket.

Thousands of construction workers joined a one-day strike.

The group organized a one-day training course on the skills needed to respond to signs of health challenges, including depression and anxiety.