If you're not married, the Internal Revenue Service only offers two ways to claim someone as a dependent: claim them as a qualifying child or as a qualifying relative. Since your significant other won't meet the relationship test required to claim him as a qualifying child, you must meet the qualifying relative tests to claim him as a dependent.
You must pay for more than half your significant other's support during the year to claim him as a dependent. Support includes just about all costs of living -- housing, utilities, food, education, transportation and even entertainment. For example, if your significant other lives with you, the value of rent and utilities counts as support you provided.
Gross Income Limit
No matter how much you pay of your significant other's expenses, you can't claim him as a dependent if his gross income exceeds the annual limit. The limit equals the value of a dependency exemption, or $3,800 as of 2012. Gross income counts as any income that isn't excluded from tax -- before any deductions are taken. For example, if your significant other has $4,000 in taxable income, you can't claim him even if he has $4,000 or more in deductions and he doesn't use any of it to support himself.
Member of Household
Since your significant other isn't related to you, he must live with you as a member of your household for the entire year. Short-term absences, such as business, vacations or medical care are allowed. In addition, your relationship can't violate local law. Say your significant other's divorce isn't finalized yet and your locality makes it illegal for him to be living with you: In that case, you can't claim him as a deduction.
Not a Qualifying Child
Finally, you can't claim your significant other if he could be claimed as someone else's qualifying child, even if that person doesn't claim him. If he's over 24 years old, there's no way for him to qualify unless he's permanently disabled. For example, say he's 23 and a full-time student. If his parent could still claim him as a dependent child, you can't claim him even if his parents don't.
Mark Kennan is a writer based in the Kansas City area, specializing in personal finance and business topics. He has been writing since 2009 and has been published by "Quicken," "TurboTax," and "The Motley Fool."