The following charts summarize the article forms and noun spelling changes across all four cases. What you need to memorize is the "range of meanings" of each article. For example: Whenever you encounter der, you need to know that you are dealing with either nominative masculine, dative feminine, genitive feminine, or genitive plural. This reading skill is sometimes going to be crucial for understanding the structure of German sentences.
|Dative||dem||der||dem||den + n|
|Genitive||des + s/es||der||des + s/es||der|
|Dative||einem||einer||einem||keinen + n|
|Genitive||eines + s/es||einer||eines + s/es||keiner|
- das and ein always indicate singular.
- des and eines are unique to singular genitive.
- dem and einem are unique to singular dative.
- die with nouns ending in –en is always plural.
Difference from English Usage
As you can see, German definite articles – in all their variety – carry a lot more information than does our one-size-fits-all, English "the." Accordingly, German uses definite articles more often than English does. This is particularly important for you to consider when a German sentence makes a universal statement. In English we signal a universal statement by avoiding "the" and/or using plural forms of nouns. German, however, often still needs the noun articles in order to clarify the sentence syntax. So it is up to you to interpret whether a statement is universal or not from the context and sense of the sentence.
Der Katzenschwanz ist ein Indikator für die unterschiedlichen Stimmungen der Katze.
The tail of a cat is an indicator of the various moods of a cat.
[or:] Tails of cats are indicators of the various moods of cats.
Die Freiheit der Meinung erlaubt aber nicht die Verächtlichmachung von Religionen.
Freedom of thought does not, however, permit the disparagement of religions.
The second example is a quotation from an online discussion forum in Germany. Your own knowledge of English tells you that translating the first phrase as "The freedom of the thought" would be inappropriate (because it doesn’t make sense, right?).
Informal person references
Similarly, German speakers may use definite articles with proper nouns or specific individuals (which we don’t do in English) in order to clarify sentence syntax. This usually occurs in more informal situations. For example:
Nein, Willi, das gehört der Mutter.
No, Willi, that belongs to Mom (or: to your mother).
Dem Karl verdanke ich die blauen Flecken hier.
I owe these bruises here to Karl.
Respect the use of articles
The reverse is not true, however. You must always understand a German noun that has no article just as you would an English noun that has no article (like Religionen in the earlier example above).
Finally, do not over-apply this rule. When inclusion of the definite article in German does make sense to carry over in to your English translation, you must do that. Imagine if the German sentence had omitted the definite article: if that would give you a different meaning, then clearly you need to respect the fact that the German sentence chose to include the definite article.