My house has a crawl space and every time we get a heavy rain I have several inches of water under the house. How do I fix this?
The fact that you only get water when it rains proves that you are dealing with a drainage issue, and not a high water table or other problem. A house has to have proper drainage to keep water away from the foundation.
Start with good gutters. Properly designed gutters should have at least one downspout for every 600 to 800 square feet of roof surface. Gutters must be clean, because dirty gutters fill up and the water overflows directly to where you don't want it: near the foundation. It's also important to make sure the ends of the downspouts are extended to discharge at least four to six feet from the foundation.
The soil around your home should slope downward six inches over the first four feet from the foundation wall. Thereafter, it can be graded more gradually but should never allow water to run back toward the house. If grading needs improvement, use clean fill dirt (not topsoil) to build up the soil around your house.