once

• Once 是引导时间或条件状语从句的连词，表示“一……就……”“当……时”，说明一个事件紧接着另一个事件发生；也可表示“曾经”“一次”，强调只发生过一次。

Once is a subordinating conjunction used to introduce a clause that indicates a condition or a single occasion when something happens. It often means "as soon as" or "when" and shows that one event will follow immediately after another. It can also express that something happens only one time or on a single occasion. As a conjunction, it is commonly used in conditional or time clauses.

1. Once as a time/condition conjunction (as soon as)B1formal

   Used to introduce a clause meaning "as soon as" or "when" something happens, often implying immediacy. The action in the main clause occurs right after the event in the once-clause. It is frequently used with present simple in the subordinate clause to refer to future events.

   时间/条件连词（一…就）

   • Once you finish your homework, you can watch TV.
   • Call me once you arrive at the airport.
   • Once the rain stops, we will go for a walk.
   • She promised to help once she has more free time.
   • Once I understand the problem, I can solve it quickly.

2. Once meaning "on one occasion" or "only once"B1formal

   As a conjunction or adverbial conjunction, it can emphasize that something happens only a single time or on one specific occasion. It highlights singularity or a unique instance rather than repetition.

   “一次”或“仅仅一次”的含义

   • Once you make a mistake like that, it's hard to forget.
   • Once in a lifetime you get such an opportunity.
   • He visited Paris only once in his life.
   • Once you try this dish, you'll want to eat it again.
   • Don't worry — it only happens once.

3. Once in conditional or hypothetical contextsB2formal

   Used in conditional sentences to mean "if ever" or "if at any time," often carrying a sense of finality or warning. It can also appear in structures expressing a completed single action that changes the situation permanently.

   用于条件或假设语境

   • Once you sign the contract, you cannot change your mind easily.
   • Once the decision is made, we have to stick to it.
   • Once he starts talking, it's difficult to stop him.
   • Once the door is locked, no one can enter.
   • Once lost, trust is very hard to regain.

• 基本数词，表示数量一、单个的人或事物；最小的正整数。

The cardinal numeral representing the quantity of a single unit; the first positive integer; the multiplicative identity in arithmetic where any number multiplied by one remains unchanged.

1. Cardinal numeral: the number 1A1formal

   Used to denote a single person or thing; the first positive integer; the quantity representing a single unit in counting, mathematics, and everyday usage. Serves as the multiplicative identity in arithmetic.

   基数词：数字1

   • I have one apple.
   • She is one year old.
   • One plus one equals two.
   • There is only one way to solve this.
   • He scored one goal in the match.