Answer: This is actually a very complex question. If a large area is constantly wet there is a severe drainage issue, either due to the soil not draining well and/or due to the overall drainage pattern directing excessive runoff to your yard from up hill.
I should mention that if your yard is literally a wetland or adjoins a wetland, there may be legal restrictions on what you can do. In general too you can't redirect runoff from your property in such a way that it floods a neighboring property.
You might be able to work with a professionally trained and degreed landscape architect to adjust the drainage pattern for the property, or to at least create a deck or patio area that is usable. Your local county extension might also be able to direct you to local resources with regard to the drainage in your neighborhood or local area. I hope this helps you get started in figuring out what can be done.
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