Definite and indefinite article

Definite article

In German, definite article is placed before noun if:

• something has already been mentioned (Der Opa liegt im Krankenhaus);
• people or objects it is used to refer to are unique. (Das Universum ist groß);
• a noun is used with an adjective in superlative degree (Superlativ) or with an ordinal numeral (Der beste Sportler in der Mannschaft; Morgen ist der fünfte Januar);
• a noun is used to refer to the days of the week, the names of months or seasons (der Donnerstag, der April, der Winter).

Declension of definite articles

Indefinite article

In German, indefinite article is used:

• if an object or a person is not identified, unknown or used for the first time (Da läuft ein Tier);
• to refer to a particular member of a group or class (Der Wolf ist ein Tier);
• to draw a comparison (Er schweigt wie ein Fisch).

Declension of indefinite articles

Zero article

In some cases, the article in German can be omitted, that is, it becomes a zero article. Article is substituted by other parts of speech if: 

• substitute words are demonstrative, possessive, indefinite, or negative pronouns (Mein Vater liest gerne);
• the noun is a proper noun or the name of a profession (Das ist meine Freundin Susi. Sie ist Lehrerin);
• the noun is in Genitiv and is atributive/a word is preceded by a noun in Genitive that is atrributive (Das ist Georgs Haus);
• Article is omitted if nouns are used without any attributes or specifications, but it is required if they are present:
• when nouns are uncountable (Maria braucht Geld; Sie hat Hunger; BUT: Maria braucht das Geld nicht; Das war der größte Hunger, den er je hatte);
• when nouns refer to substances (Er trinkt Kaffee ohne Zucker und ohne Milch);
• no article is used in fixed expressions (aus Freude; auf Schritt und Tritt);
• Articles are always omitted when they are used with certain prepositions, e.g. the preposition ohne (ohne Angst; ohne Wind; ohne Wut).

Exceptions

If the name of a nationality (Engländer, Ukrainer, Österreicher) or a profession (Lehrer, Koch, Fahrer) or ‘a core job cluster’, which are related to profession/ denote a profession (Student, Schüler, Volleyballer) in such sentences as „Ich bin Österreicher“, „Sie ist Lehrerin“, „Er ist Student“, the names of these groups are used without any article.

BUT: if only one representative of the above group is referred to (Ich bin nur ein Lehrer) or an indefinite representative of this group is mentioned (Ein Ukrainer und ein Engländer waren hier gestern unterwegs), the article should be used.