Three little words that are so similar but so different (and cause so much confusion 
)
But let’s have a closer look at each one of them:
“am” is actually a combination of the words “an dem” (at the). It’s used to describe being at a certain place, which means it’s always linked with Dativ. Would you like to see some examples? Here they are!
-
Ich warte am Bahnhof auf dich. (I’ll wait for you at the main station.)
-
Wir grillen heute am See! (Today we are barbecuing at the lake.)
-
Du arbeitest am neuen Projekt. (You’re working on the new project.)
Very similar to “am” is “im”, which is composed of “in” and “dem” (“in” + “the”). It shows that you are in a certain place or indicates a certain time. Let’s put it into practice!
-
Im Haus ist es sehr warm. (It’s very warm in / inside the house.)
-
Sarah taucht gern im Roten Meer. (Sarah likes to dive in the Red Sea.)
-
Im Winter gibt es in Kanada viel Schnee. (There’s a lot of snow in winter in Canada.)
The preposition “um” is different from the other two. “um” has two meanings. It refers to a certain time (“at” 2 pm) and can also mean “around” when talking about a place. Have a look at the following examples:
-
Wir treffen uns um 18 Uhr. (We’ll meet at 6 pm.)
-
Ich habe heute um 14:30 Uhr einen Arzttermin. (Today I have a medical appointment at 2:30 pm.)
-
Die Stühle stehen um den Tisch. (The chairs are around the table.)
-
Morgen laufen wir um den See. (Tomorrow we’ll walk around the lake.)
Great! Now you know how to use those three tricky little prepositions. What about forming a sentence where all three prepositions appear? Just try it! 
So I consulted my good friend Google. I still don’t know if “um” is always a preposition or considered some other word class when you use it to give a reason, but I stumbled upon a very informative site which might be interesting for students interested in “um”: 
