Tax rates on salaries start from 33% and can go up to the maximum of 52%. A part of these taxes consists of social security premiums. The fiscal year runs from 1 January to 31 December. Around the first week of February, the tax authorities (Belastingdienst) will send out tax return forms.
If you are employed by the UvA, tax is withheld from your salary each month. You will most likely not receive a tax return form. You may, however, have made tax deductible expenses during the year and could be entitled to a tax refund. So it’s often a good idea to request for a form yourself.
If you are not employed by the UvA, your income may or may not be subject to taxation. Please note that in case you received income which should be taxed but still hasn't been, you are legally obliged to request a form and file a tax return.
In general, tax returns should be submitted by 1 April. Extensions are possible if you cannot meet the deadline. You can file your taxes yourself via a digital form, which can be downloaded from the Belastingdienst website. This form is, however, only available in Dutch and you will first need to apply for a digital identification number called DigiD. The other option is to ask for the help of a tax accountant or adviser.
The Belastingdienst website does have a section in English with more information about Dutch income tax regulations and filing taxes.