once

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once用作連詞時(shí)可引導(dǎo)時(shí)間狀語(yǔ)從句和條件狀語(yǔ)從句。用於前者時(shí)意爲(wèi)“一旦…就…”,所引導(dǎo)的從句多很短; 用於後者時(shí)意爲(wèi)“衹要”,這種條件從句似帶有時(shí)間條件的含意。
once引導(dǎo)的從句中有些部分可以省略。

If something happens once, it happens one time only.
Once is also a pronoun.
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.
If something was once true, it was true at some time in the past, but is no longer true.
If someone once did something, they did it at some time in the past.
If something happens once another thing has happened, it happens immediately afterward.
If something happens all at once, it happens suddenly, often when you are not expecting it to happen.
If you do something at once, you do it immediately.
If a number of different things happen at once or all at once, they all happen at the same time.
For once is used to emphasize that something happens on this particular occasion, that it has never happened before, and may never happen again.
If something happens once again or once more, it happens again.
If something happens once and for all, it happens completely or finally.
If something happens once in a while, it happens sometimes, but not very often.
If you have done something once or twice, you have done it a few times, but not very often.