When would one use “im” and “am” rather than “in dem” and "an dem?

Question

I'm new to German and I get a bit confused about when it's "dem" or "im" or "den" rather than "in der" or "in dem" and so on.

Answer 1

In informal speech uncontracted expressions like in dem Haus, an der Strasse etc. may occur, but always in a meaning like in diesem Haus or an dieser Strasse.

- "In welchem Haus wohnt er?"
- Er zeigte auf das linke Gebäude und sagte: "In dem Haus."

Answer 2

Und noch mehr:

zu + der = zur (Ich gehe zur Schule)

Aber die Frage war "when would one ..." :

Praktisch immer, ausser in sehr förmlichen Texten, wie in Gesetzestexten oder behördlichen Schreiben.

Answer 3

In written German, use only the contractions enlisted by Phira.

Use contractions:

• in patterned speech

  Etwas zum Besten geben, zur Schule gehen

• in superlatives

  Hans ist am größten.

• If the noun on which the preposition is related has already been introduced or needs for other reasons not to be emphasized (e.g., if it is not important which one it is), Better to use a contraction leads to a less "emphasizing" character of the expression:

Ich bin zum Bäcker gegangen.

But

An dem Tag habe ich schon etwas vor.

If you are in doubt, I would advise not to use a contraction as they tend to sound a little "uncouth" if overused.

Answer 4

In many languages, prepositions and articles are sometimes contracted.

Obligatory contractions:

• an + dem + noun → am + noun
• in + dem + noun → im + noun
• zu + dem + noun → zum + noun
• zu + der + noun → zur + noun

Contractions that may occur in informal speech:

• vor + dem + noun → vorm + noun

As for choosing between different cases:
If the case comes after a verb, you have to know what case the verb demands.

This corresponds to the knowledge of deciding if the correct English sentence is "I touch you." or "I touch to you." and very often, the construction with "to" corresponds to dative in German and without it to accusative in German. In the light of the next phrase it can be more helpful to think of the action being done "for" someone when the dative is used.

For prepositions again, you should learn what case they demand, but an important rule is: If the meaning is where something is, then the object has to take the dative, if the meaning is where it moves to, then the object has to take the accusative. Note that this deviates from the correspondence with "to" in the previous paragraph.

It would be good to ask a more specific question if you want to know more.