Medicare is a crucial part of retirement, yet 72% of Americans say they wish they better understood how the program works, according to a survey from Nationwide.

Healthcare expenses can be significant in retirement, so not fully understanding Medicare could be a costly mistake. And these three mistakes in particular could throw off your entire retirement plan.

1. Assuming Medicare will cover all your healthcare costs

Medicare will help cover some of your medical expenses in retirement, but it doesn't cover everything. For one, you'll still be responsible for all premiums, deductibles, co-insurance, and co-pays. With Medicare Part A, you typically won't pay a premium, but you will face a deductible of $1,408 per benefit period. With Part B, the standard premium is $144.60 per month with a deductible of $198 per year.

But keep in mind that Medicare Parts A and B do not cover prescription drugs or routine vision and dental care. For that type of coverage, you'll need to enroll in Medicare Part D or a Medicare Advantage plan at an additional cost.

In addition, Medicare typically doesn't cover long-term care, which can be incredibly expensive. The average semiprivate room in a nursing home costs just over $6,800 per month, according to the U.S. Department of Health and Human Services. Without help from Medicare, you may be left to foot this massive bill on your own. Before you retire, it's a good idea to factor these costs into your retirement plan or consider signing up for long-term care insurance.

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