1. Explain it simply
要是 introduces a condition ('if…'), placed at the start of that clause.
就 goes right before the verb in the second clause to mark the result ('then…').
2. Show examples
要是明天下雨，我们就不去了。
Yàoshi míngtiān xià yǔ, wǒmen jiù bú qù le.
If it rains tomorrow, we won't go.
要是 + condition, 就 + result
要是你有时间，就来找我吧。
Yàoshi nǐ yǒu shíjiān, jiù lái zhǎo wǒ ba.
If you have time, come find me.
A gentle suggestion built on a condition
要是不懂，就问老师。
Yàoshi bù dǒng, jiù wèn lǎoshī.
If you don't understand, ask the teacher.
要是 can drop the subject when it's understood
3. Share a tip
就 is optional in casual speech, but keeping it makes the if-then link clearer, especially in writing.