The system for German income taxes for individuals does not make a difference if you are a German or a foreigner. There also is no basic rule that a German has to pay his taxes no matter where he lives in the world (no world income taxation as it is used in the US). On the other hand this means that also foreigners have to pay taxes in Germany even if they have to pay taxes in their home countries as well. Please see the details for german income taxes for individuals in Germany in the following articles:
As an individual the deadline for filing your tax declaration is 31st of May. If you have a tax consultant working for you the deadline is end of December if the tax office does not ask you to file the declaration earlier. This might for example be the case if you start your business in Germany.
You will have to file the tax declaration except if you are an employee with no other income and tax class one. The basic rule is that the only cases where you are not obliged to file the declarations will be the cases where you can expect a refund. So you should always at least let a tax consultant check what refund is to be expected so you will not pay more taxes than you have to.
You can also file the declarations for former years if you expect a refund. Typical cases for refund situations are higher costs for movement or if you have a double household (=you kept your home and rented an appartment where you work).
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