You are required to file a Dutch personal income tax return if one has been issued to you. When no personal income tax return has been issued to you and you are a resident tax payer, then you are required to file a personal income tax return yourself if you had multiple employers, that is more than one. Or when you had freelance income or other income, not being a hobby, that needs to be reported.
You also have to file a tax return if you already were paid through a preliminary refund the mortgage refund, in order for the tax office to check if that refund balances with the mortgage details.
Furthermore you are required to file a personal income tax return if you have in world wide assets more than about EUR 21.139 per tax payer. For tax partners the amount above which you need to pay tax is EUR 42.278.
When are you regarded a resident tax payer? You are regarded a resident tax payer when you have your central point of life in the Netherlands. Your central point of life is determined by your partner, children, mail, friends, work etc. Hence, if you are single, but you have work in the Netherlands, then you are regarded a resident tax payer. However, if you are not single and your partner lives abroad, you might be regarded a non resident tax payer. Non resident tax payers do not pay tax over the days they have not spend physically in the Netherlands.
You are also regarded a non resident tax payer if you have real estate situated in the Netherlands. Based on the tax treaties, real estate is taxed in the country where it is situated. In your home country you need to report it as well, as being part of your world wide assets, but your home country will provide a double taxation relief. As non resident tax payer with real estate situated in the Netherlands you are obliged to file a non resident income tax return. If the real estate is rented, then the rent is exempted from taxation, as is the mortgage interest paid. Taxed is the economic value determined by the county minus the debt related to the real estate. The economic value can under circumstances be reduced.