Breakdown of yàoshi míngtiān hái bù hǎo, tā jiù děi zài yùyuē yīshēng.
Questions & Answers about yàoshi míngtiān hái bù hǎo, tā jiù děi zài yùyuē yīshēng.
要是 means if. It introduces a condition:
要是 + condition, (就) + result
In everyday speech, 要是 is very common and a little more conversational than 如果. In this sentence, you could also say:
如果明天还不好,她就得再预约医生。
That would mean basically the same thing.
Chinese often omits a repeated subject when it is already obvious from context. So:
要是(她)明天还不好,她就得再预约医生。
Both are natural. The sentence leaves out 她 in the condition clause because the second clause makes it clear who we are talking about.
For learners, it may feel easier to include the subject at first, and that is completely fine.