once

once
once 一旦; 一度
Once you begin, you must continue.
一旦开了头,你就必须继续下去。
Once you have taken the examination, you'll be able to relax.
你一旦考过试,就可以轻松一下了。
Once you have promised you must do it.
你既已答应,就应该去做。
Once inside her bedroom, she began to cry.
一走进卧室,她就哭了。

详解：

1.once用作连词时可引导时间状语从句和条件状语从句。用于前者时意为“一旦…就…”,所引导的从句多很短; 用于后者时意为“只要”,这种条件从句似带有时间条件的含意。
2.once 和as soon as 两者均为从属连词，引导时间状语从句，都表示主句的动作紧跟在从句的动作之后发生。
once:除表时间外，还表示条件，一般译作“一旦......就......”。
他一旦下定决心，就决不会放弃（本句不可用as soon as）。
Once he makes up his mind，he’ll never give it up.
as soon as:强调的只是时间，译作“刚......就......”或“一......就......”。
我一到北京就写信给你。
As soon as I get to Beijing，I’ll write to you.

nevertheless<-->on condition that