如果⋯⋯的话
rú guǒ …… de huà
Expressing "if" with "ruguo… dehua"
的话 closes the if-clause and makes the condition sound more tentative.
如果 + Condition + 的话，(就) ⋯⋯
How it works
如果明天下雨的话，我们就不去了. The 的话 is optional, and adds a note of "supposing that".
如果 itself can be dropped once 的话 is there: 明天下雨的话，我们就不去了.
要是 is the more colloquial substitute for 如果 and works in all the same shapes.
Examples
rú guǒ míng tiān xià yǔ de huà, wǒ men jiù bú qù le
如果明天下雨的话，我们就不去了。
If it happens to rain tomorrow, we won't go.
yǒu wèn tí de huà, kě yǐ wèn wǒ
有问题的话，可以问我。
If you have any questions, you can ask me.
yào shi nǐ bù xiǎng qù, jiù bié qù
要是你不想去，就别去。
If you don't want to go, then don't.