Ever thought it makes a linguistic difference? While in English it doesn't, German is a little different... again. There are two words in German and they're actually pretty different. The heroes of this article are nach and zu (well... "in" is sth. like a sidekick I guess).
nach and zu both mean "to" but they're not interchangeable.
You use nach when going to a specific city, country or continent. (except for Antarctica... in case this is a continent to you) Still you should be careful here, because you only use "nach" when using the name of that place. So you go "nach London" but "zu" a city. Same counts for continents and countries. Always when not mentioning the specific name but just saying "this country" or so... you'll have to use zu.
Using "zu" is pretty easy because you can always use it when not using "nach". Well... always except when emphasizing that you enter a building. Then you will use "in". When going downtown, you'll also have to use "in". In German this would be: "Ich gehe in die Stadt." or "Ich gehe in die Innenstadt." (Stadt = city; Innenstadt = city center or "inner city")
I used "zu der" for the sake of simplicity. Actually "zu der" becomes "zur", so people would normally say: "Ich gehe zur Bibliothek."
So when going to the library, in and zu are basically interchangeable, since you're usually going to enter the library when going to it. In case you're going there to meet a friend of yours in front of the library, then "zu" is the right choice.
"Ich gehe zu der Bibliothek um dort einen Freund zu treffen."
"I'm going to the library to meet a friend (there)."
Some Germans even don't get the difference between "zu" and "nach". They'll say stuff like "Ich gehe nach Aldi." (Aldi is a popular supermarket in Germany... since supermarkets are no countries, no matter how popular they are... "zu" is actually the right word) If you ever hear a German saying something like that to you, you have only one choice. Start screaming and run for it - don't let him ruin your grammar! ;)
Expert Tip:
Some word-combinations merge into one single word. This is the case with the following words.
zu der = zur (Ich gehe zur Bilbiothek. / "I go to the library.")
zu dem = zum (Ich gehe zum Supermarkt. / "I go to the supermarket.")
in das = ins (Ich gehe ins Haus. / "I go into/enter the house.")
While zu der and zur (etc.) have the exact same meaning, it's actually best to use those compound words (zur/zum/ins) whenever possible. People are far more likely to say "Ich gehe zum Supermarkt." instead of "Ich gehe zu dem Supermarkt.".
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