我要是知道有那个会，就去参加了。
If I had known about the meeting, I would have attended.
▼点击显示答案
had英 /hæd/|美 /hæd/
known英 /nəʊn/|美 /noʊn/
解释
had known 是过去完成时，用在 if 条件句中表示对过去情况的假设（实际上当时不知道）。know 的过去分词是 known，与 had 一起构成过去完成时。与主句的 would have attended 搭配，构成第三类条件句，表达对过去未实现情况的遗憾。
If I had known about the meeting, I would have attended.
had known 是过去完成时，用在 if 条件句中表示对过去情况的假设（实际上当时不知道）。know 的过去分词是 known，与 had 一起构成过去完成时。与主句的 would have attended 搭配，构成第三类条件句，表达对过去未实现情况的遗憾。