The second injection should only have been given once the first drug had taken effect.

第2针本应只在第1针见效后才能注射。

《柯林斯英汉双解大词典》

Once the first avowal has been made, “I love you” has no meaning whatever.

一旦有了第一次的告白，“我爱你”就再没有任何意义。

youdao

"Once the first wave hits the coastline, it gets very complicated," Dr. Geist said.

“一旦第一波波浪到达海岸线，情况就会变得非常复杂，”Geist博士说。

youdao

The agreement,once it takes effect, will be the first under international law to target just this problem.

VOA: special.2009.12.08

Once again the row player is the first payoff and the column player is the other payoff.

这次还是第一个收益是我的,另一个收益是我对手的

耶鲁公开课 - 博弈论课程节选

And they completely change once your first paper is due or the midterms arrive.

要写第一篇论文，时，情况彻底改变,或期中考试来临时。

哈佛公开课 - 幸福课课程节选

Moreover, once the first global tax is imposed, the floodgates will be opened for similar proposals.

FORBES: Should International Bureaucracies Get Taxing Powers or Direct Funding?

All of that pre-parade analysis went out the window once the first band marched past me.

FORBES: I Can't Keep My iPhone in My Pants

Once the first account is paid off, reward yourself and your family with a day together doing something fun.

FORBES: Dump Your Debt and Reward Yourself