Your original sentence: Mountains are like huge folds in the Earth's rocky coat that once risen stand for centuries.
Corrected sentence (keeps your words): Mountains are like huge folds in the Earth's rocky coat that, once raised, stand for centuries.
Simple version (easier to understand): Mountains are like huge folds in the Earth's rocky skin. After they rise, they can stand for centuries.
Why we changed it:
- "Once risen" sounds awkward because "risen" needs a helper or a subject. Saying "once raised" or "once they rise" is clearer.
- We put commas around "once raised" because it is extra information added to the sentence.
- "Rocky coat" is okay as a picture, but "rocky skin" or "rocky crust" are simpler words a child might understand better.
If you want, I can make the sentence even shorter or help you write a new one that sounds like how you speak.