Live or stay?
Whether you permanently live in the Netherlands or not: the Dutch law is applicable because of the employment relationship.
When do you live in Netherlands?
You can only live in one country. You live in the Netherlands if you have more social, economic and legal connections with the Netherlands than with Spain. For example: you live in the Netherlands as your family also lives here.
Does Your family lives in Spain? Then you do not live in the Netherlands, but stay there because of work.
Domestic or foreign taxable person?
If you do not ive in the Netherlands but staying there for work, you are a foreign taxable person. This means that you pay incometax only over your Dutch income in the Netherlands.
As a foreign taxable person, you can choose to be treated as a domestic taxpayer. You'll get discounts and deductions on income tax.
If you have no deductions and your partner works? Then there is no advantage to be classified as a domestic taxpayer.
You pay incometax in the Netherlands. The higher the salary, the more incometax. The employer keeps the incometax on wages and pays the incometax to the tax authorities in the Netherlands. In the calculation of the incometax, the employer shall take into account taxcredits.