Most German language courses teach their students to say, “Guten Morgen!” “Guten Tag!” or “Guten Abend!” Since these greetings are formal, they will always be appropriate. However, Germans passing on the street may simply say, “Tag!” or one of its regional equivalents like “Grüß Gott!” or “Servus!” or “Moin!” In my part of Germany, where there are many Americans, they also may say, “Hallo!” I think that’s because they’ve gotten used to us saying, “Hello!” all the time.
Germans use “Hallo!” to draw attention to something, the way we use the word “Hey!” A German might say, “Hallo, Sie haben etwas verloren!” (“Hey, you’ve lost something!”) to let you know that you’ve accidentally dropped an item. If your front door is open, a German visitor might call, “Hallo!” while walking inside in the same tone that we would use to say, “Hello? Is anyone there?”
Germans also use “Hallo!” to express pleased surprise, particularly when they’ve been looking for something and suddenly spot it. In this case, “Hallo!” is like our “Aha!”