So you've retired and claimed Social Security benefits. Congrats: You've hit a major life milestone. But you can't necessarily just sit back and enjoy your checks while having the freedom to do whatever you want. Certain things you do may affect how much of your benefits you get to keep. 

If you don't want to be caught off guard by unpleasant surprises that impact your household income, make sure you know these two key rules. 

Social Security card with money sitting on top of it.

Image source: Getty Images.

1. When your benefits become taxable

Many seniors aren't aware that the IRS could take part of their benefits, but it's definitely possible. So prepare to adjust your budget and pay your tax bill on time. 

About half of all Social Security retirees owe taxes on their benefits, which kick in at $25,000 in provisional income for single filers and $32,000 for joint filers. Once you hit these income thresholds, up to 50% of benefits could be taxed. And once your income reaches $34,000 as a single filer or $44,000 as a married joint filer, up to 85% of benefits could be taxed. 

Not all income is factored in. Half your Social Security benefits, some nontaxable income, and all taxable income are added up to determine how much you'll pay. 

Depending on where you live, it's also possible you could owe state taxes. There are 13 states that tax your benefits, so learn the rules if you live in one of them. 

2. What happens if you work when receiving benefits

Claiming Social Security doesn't mean you must give up work, but earning a paycheck could affect how much of your benefits you get to keep. This won't be an issue if you've already reached full retirement age (FRA). But for anyone who hasn't hit that milestone, you'll forfeit some benefits once your earnings from work exceed a certain limit

If you'll be under FRA during the entire year, you'll lose $1 in benefits for every $2 earned above $18,240 in 2020 (it's $18,960 in 2021). If you'll hit FRA sometime during the year you're working but haven't reached it yet, you'll lose $1 in benefits for every $3 earned above $48,600 in 2020, or $50,520 in 2021. 

But the benefits you don't receive for this reason aren't necessarily gone forever. The Social Security Administration (SSA) withholds whole checks to account for them. So, for example, if your check is $1,200 per month, and you're forfeiting $2,400 in benefits due to working, you'd lose two entire checks. But when you reach full retirement age, the SSA credits back your early filing penalty for any months you didn't get money.

It takes some time to break even for the forfeited benefits, and not everyone lives long enough to do so. For that reason, working while receiving benefits could potentially mean less lifetime income from the SSA. It also means less income in the short term since you won't get both your paycheck and the full benefit amount.