Preterit
a. 过去的\\nn. 过去时
✅ 同义词
🗣️ 情景例句
It's a problem, isn't it?
这确实是个问题，不是吗？
What is happening over there?
那里发生什么事情？
it is extremely difficult to estimate the true value of the services which people perform for us.
但要估算别人为我们为所提供的服务的价值却是极其困难的。