The endings of certain words change depending on how they are used in a sentence. There are four categories called "cases." The nominate case indicates that something is the subject of a sentence. The accusative case applies to direct objects, while the genitive case indicates possession or ownership. The dative case applies to indirect objects which receive the direct object or an action. For example, in the sentence, "Jack threw the ball to Jane's dog," the noun Jack is nominate, the ball is accusative, Jane is genitive and the dog is dative.
Definite articles precede a noun that is a specific thing. In English there is only one definite article: "the." German is more complicated and has three, depending on the gender of the noun. In the nominative case "der" precedes masculine nouns as a definite article, "das" is paired with neuter nouns, and "die" goes with feminine and plural nouns. In the accusative case the masculine "der" becomes "den." In the genitive case, "der" and "das" become "des" and "die" becomes "der." In the dative case "der" and "das" become "dem," the feminine "die" becomes "der" and the plural "die" becomes "den." Other words such as "dieser," "welcher," "jeder" and "jener" (this, which, each and that) change their endings in the same way.
Indefinite articles precede nouns that are unspecific. In English the indefinite articles are "a" and "an." In the nominate case the German indefinite articles are "ein" for masculine and neuter nouns and "eine" for those that are feminine. Plural nouns do not have an indefinite article. Indefinite articles change their endings based on case and gender in the same way as definite articles. In the accusative case the masculine "ein" becomes "einen." In the genitive case "ein" becomes "eines" and "eine" becomes "einer." In the dative case "ein" becomes "einem" and "eine" becomes "einer." Possessive adjectives behave the same way as indefinite articles.
Most German nouns ending in -en, -el, -ling, -ner, -ismus, -ig, ich, or -er are masculine. Time expressions (days of the week, months, seasons) and phrases describing weather are usually masculine. German nouns that end in -heit, -ie, -ik, -age, -ei, -ion, -itis, -keit, -ur, -schaft, - tät and -ung are often feminine, as well as almost every noun that ends in -e. Cardinal numbers and plantlife are also typically femine. Nouns which end in -ett, -chen, -lein, -il, ium, -ma, -ment, -nis, -tel, -tum, -um and -o are normally neuter. Infinitive verbs used as nouns, colors, countries and towns also tend to be neuter.