If something was once true, it was true at some time in the past, but is no longer true. The culture minister once ran a theatre I lived there once myself, before I got married The house where she lives was once the village post office My memory isn't as good as it once was
emphasis For once is used to emphasize that something happens on this particular occasion, especially if it has never happened before, and may never happen again. For once, dad is not complaining His smile, for once, was genuine
You use once with `a' and words like `day', `week', and `month' to indicate that something happens regularly, one time in each day, week, or month. Lung cells die and are replaced about once a week We arranged a special social event once a year to which we invited our major customers
Once upon a time is used to indicate that something happened or existed a long time ago or in an imaginary world. It is often used at the beginning of children's stories. `Once upon a time,' he began, `there was a man who had everything.' Once upon a time, asking a woman if she has a job was quite a straightforward question
If something happens once in a while, it happens sometimes, but not very often. Earrings need to be taken out and cleaned once in a while. = occasionally
If someone once did something, they did it at some time in the past. I once went camping at Lake Darling with a friend We once walked across London at two in the morning Diana had taken that path once