All rules about how much Medicare Part A pays depend on how many days of inpatient care you have during what is called a "benefit period," or spell of illness. The benefit period begins the day you enter the hospital or skilled nursing facility as an inpatient and continues until you have been out for 60 consecutive days. If you are in and out of the hospital or nursing facility several times but have not stayed out completely for 60 consecutive days, all of your inpatient bills for that time will be figured as part of the same benefit period.
Medicare Part A pays only certain amounts of a hospital bill for any one benefit period -- and the rules are slightly different depending on whether the care facility is a hospital, psychiatric hospital, or skilled nursing facility, or whether care is received at home or through a hospice.
All people covered by Medicare Part A must pay an initial amount before Medicare will pay anything. This is called the hospital insurance deductible. The deductible is increased every January 1.
You can get all the details in our article on Part A medical coverage.
For gaps in what Medicare Part A covers, including deductibles and co-insurance amounts, see Nolo's article Medigap: Covering the Gaps in Medicare.