If I claim my partner as a dependent does she have a lower tax liability and how does that help/hurt my tax liability?

A significant other can be claimed as a dependent if they pass some of the same tests used to determine if your child or relative can be claimed as a dependent.  First, your significant other cannot be claimed as a dependent if they are eligible to be claimed as a dependent on another tax return. Whether your boyfriend or girlfriend is being claimed is irrelevant, it’s the eligibility that matters.

She must pass the following test to qualify as your dependent.

1. Cannot be the qualifying child of anyone else.                             
2. Lived with you all year.
3. Gross income for the year must be less than $4,050.                           
4. You must have provided more than half of her total support during the calendar year.

Each dependent (such as your partner) lets you claim a dependency exemption, reducing your taxable income by $4,050 per dependent for the tax year 2017 which should result in a lower tax liability.  However, your partner does not get to reduce her income by $4,050 due to the loss of her personal exemption, which will result in a higher tax liability for her.  

For more information about claiming your partner/girlfriend/boyfriend/significant other as a dependent, please see the link below:

https://blog.turbotax.intuit.com/tax-deductions-and-credits-2/can-i-claim-my-girlfriend-or-boyfriend-as-a-dependent-21025/