"I will call the police."
Translation:Ich werde die Polizei rufen.
"Ich rufe die Polizei" ist gewiss richtig. Wieso halten Sie es für falsch? Schlagen Sie mal die Verben rufen und anrufen im DUDEN nach.
In der Regel wird ins Deutsch das Präsens mit der Bedeutung von Futur I verwandt. Die Anwendung des Futurs in diesem Fall wäre wahrscheinlich grammatikalisch richtig, doch ginge es um eine komische Konstruktion.
Dann wäre es aber "I call the police". I WILL call the police hat mehr den Klang einer Drohung für die Zukunft - wenn ihr nicht aufhört, dann werde ich die Polizei rufen. Ich glaube also, Duo hat recht.
Ungeachtet dessen arbeitet Duo manchmal mit einem eingeschränkten Satz an möglichen Lösungen und ist träge im Hinzufügen von zusätzlichen Möglichkeiten.
First it is not quite right in English to say I call the police (simple present) to express one's intention to call the police. Further, Duo lingo proposed that the correct translation of "I will call the police" is "Ich rufe die Polizei an" instead of "Ich rufe die Polizei" that I wrote, so again Präsens. Therefore my first comment was referring to the use o verbs "rufen" and "anrufen" and not to the use of present or future tense. Of course Duolingo is not a human being and will therefore accept the construction with werden as a literal translation. However, as already mentioned, the use of Futur I to express an intention or a threat (an act with more or less immediate character) as you write is not exactly correct. Unless if one wants to emphasize that he is planning to call the police some time in the future.
I am sorry to say, but I am afraid that this is not exact. The rules for the use of Future tense 1 mentioned above are, to the best of my knowledge, valid for formal writing, as well. Future 1 is to be used in order to express a forecast for a future situation (e.g. team A will be a national champion at the end of the season) or in order to express an intention/promise for an action that is due in a future time (e.g. that you will visit your parents next month). All this stuff can be found in standard textbooks of German grammar.
In most textforms you do not use the Präsens to describe the Futur1. I can only think of something like a letter to a friend (something not really formal) where you can use it (additionally all electronic equivalents.)
Also if you use a futuric Präsens in a normal german school, then you will get it marked as mistake in nearly all cases. It just happens that people speak like this (= using the futuric Präsens) and thats why it exists. Its nearly never used (I would say never, but maybe someone does) in most text types.
Also standard grammar books for germans (in german) do not suggest the learner to use the Präsens fur Futur1 in written language.
Thanks for the extensive information. I could not know that the grammar taught in German schools could deviate from what I found as a rule in some reference grammar books (see below). Of course I have no reason to defy your input since you went to school in Germany :). Some of the grammar books where I looked up the subject are: Dreyer - Schmitt: Die gelbe aktuell, ISBN 9783193072559 S. 134-135. Hall/Scheiner: Übungsgrammatik für Fortgeschrittene, ISBN 9783190074488 S. 321-322. C-Grammatik (Schubert Verlag ISBN 9783941323117), S. 27.
I dont have that book, but I guess they might have just explained all potential usages of the Präsens without restricting it for which situations they are useful.
So I dont want to discredit this book (at least not without knowing the exact text that they wrote). I just wanted to say, if youre writing a business email, you should use Futur1 - even when your boss might not care for futuric Präsens, youre always safe with it in texts.