Yídàn nǐ mínɡbɑi, jiù huì hěn jiǎndān.
一旦你明白，就会很简单。
It's easy once you know how.

 
Notes:
一旦(yídàn): once
你(nǐ): you
明白(mínɡbɑi): understand
就(jiù): just
会(huì): will
很(hěn): very
简单(jiǎndān): easy
 
Example:
A：为什么我不明白？
     Wèishénme wǒ  bù mínɡbɑi？
     Why don't I understand?
B：一旦你明白，就会很简单。
     Yídàn nǐ mínɡbɑi, jiù huì hěn jiǎndān.
     It's easy once you know how.