Most nouns can be either singular or plural. The plural indicates that you're talking about several units of the same thing.

Example:
Ist das Bett zu hart? – Sind die Betten zu hart?
 

The definite article in the nominative and the accusative is always "die" in all three genders: masculine, feminine and neuter. The indefinite article is omitted.

Example:
Ich mache einen Test. – Ich mache _ Tests.
 

The negative article in the nominative and the accusative is "keine".

Example:
Haben Sie ein Zimmer? – Nein, wir haben keine Zimmer.
 

There are altogether five different endings to form the plural:

Some nouns stay the same in both the plural and the singular.

Example:
das Zimmer – die Zimmer (endungslos)

In such cases, the definite article "die" or the use of a determiner indicating number or amount - such as "zwei" (two), "viele" (many), or "mehrere" (several) - help to identify the plural.

Example:
Wir haben ein Zimmer. – Wir haben viele Zimmer im Hotel.

More:

In many cases, turning a noun into the plural requires not only adding an ending, but also changing a stem vowel to an umlaut: a becomes ä, o becomes ö or u becomes ü. Sometimes an umlaut change is the only indication of the plural.

Note that with some words, the final consonant is doubled before the plural ending is attached. This doesn't happen often and affects mainly words that end with -nis and -in. But it applies to most female forms of describing people because they often have -in at the end: