Millions of seniors rely on Medicare for health coverage during retirement. But the less you know about how that program works, the more you risk struggling to afford your healthcare costs under it.

Unfortunately, a recent Nationwide survey reveals a major knowledge gap on the Medicare front: A surprising 79% of future retirees erroneously think that Medicare Part B, which covers doctor visits and diagnostic services, is free to workers who pay Social Security taxes for at least 10 years.

Not only is Medicare Part B not free, but it's also just one of many things you'll need to pay for under Medicare. And the sooner you read up on what costs to expect, the better you'll be able to plan for them.

How Medicare works

Medicare is divided into several distinct parts. Part A covers hospital care and is generally free to enrollees. That means you don't pay a premium for Part A -- but it doesn't mean you don't face expenses if you land in the hospital. Currently, you'll pay a $1,364 deductible toward your expenses under Part A before Medicare kicks in to cover the rest of your services. From there, you may be responsible for coinsurance, depending on the length of your hospital stay.

Continue reading on The Motley Fool.