I have often got confused when to omit definite/indefinite articles in German. Here are some of the simple tips on when to omit definite or indefinite articles in german. This is otherwise known as zero/null article in german.

You can omit articles in the following cases..

Before nationality

Before personal name

Before most countries (except countries like die Schweiz, Die Tuerkei, Die USA)

Before a profession.

Before nouns which give `some`/`many` meanings

Definite Article

Der/Die/Das always omitted for the following cases..

a musical instrument, especially when its played.

Before musical instruments.

Er spielt  __x__ Violin.

The answer marked x which means article is omitted.

Indefinite Article

Indefinite articles (ein/kein…etc) are always omitted in the following cases…

Grammar Rule : Eine is omitted or will disappear before plural nouns, whereas keine will not.

Masculine – ein/kein
Neuter – ein/kein
Feminin – eine/keine
Plural – x/keine

Das sind _____ Lampen.

You will be tempted to put `eine` as the answer but the correct answer is there will be article omission. You can mark it as ‘x’ which means Null Artikel.