Why exactly do I owe this year I never change the thing on my W-4, and can I claim my girlfriend as a dependent, for the most part I paid most of the bills.

Q. Can I claim my girlfriend as a dependent?

A.  Probably not. Paying "most of the bills" is not enough.  More importantly, she must have less than $4400 of income (not including social security) and must have lived with you all year. 

There are two types of dependents, "Qualifying Children"(QC) and Other ("Qualifying Relative" in IRS parlance even though they don't have to actually be related). There is no income limit for a QC but there is an age limit and student status test, a relationship test and a residence test. 

A person can still be a Qualifying relative dependent, if not a Qualifying Child, if he meets the 6 tests for claiming a dependent:

1. Closely Related OR live with the taxpayer ALL year
2. His/her gross taxable income for the year must be less than $4400 (2022).
3. The taxpayer must have provided more than 1/2 his support

In either case:

4. He must be a US citizen or resident of the US, Canada or Mexico
5. He must not file a joint return with his spouse or be claiming a dependent of his own
6. He must not be the qualifying child of another taxpayer