要不是【yàobúshì】

要不是 opens a sentence with something that did happen, and the second half says what would have happened without it. It takes a noun or a whole clause.

The second half usually has 就 or 早就 and ends in 了, because it describes the outcome that was avoided or missed. 早就 adds "long since".

Only for what is true

The thing after 要不是 must be a fact. You're saying it happened, and imagining the other way round. For a real "if not" about something still open, use 如果不 or 如果没有.

• × 要不是明天下雨，我们就去爬山 → 如果明天不下雨，我们就去爬山

Thanks and blame

Because it points at a cause, 要不是 + person is a common way to thank someone, or to blame them.