"Nouns that only exist in plural form" means: the verb that is used with the noun has to be in plural:
Most nouns can be used both in singular and in plural:
Singular: ---------------- Plural:
a) The tree is green --- The trees are green
b) The mouse is grey -- The mice are grey
c) The sheep is white -- The sheep are white
--> "trees" is a regular plural form, "mice" and "sheep" are irregular ones.
Some nouns only have a plural form = verb in plural (even though there's only one thing):
c) Your new trousers are nice --- not: Your new trousers is nice
d) The shorts are too small ------ not: The shorts is too small
e) The police find money -------- not: The police finds money
Unlike English, all these nouns c-e are singular in German, i.e. used with verb in singular:
c) Deine neue Hose ist (not: sind) schön --> literally: Your new trousers is nice
d) Die kurze Hose ist (not: sind) zu klein --> literally: The shorts is too small
e) Die Polizei findet (not: finden) Geld -----> literally: The police finds money
Some nouns only have a singular form = verb in singular (even though there are more than one thing):
f) The furniture is nice --------------- not: The furniture are nice
g) The information is helpful -------- not: The information are helpful
h) The news is coming on ----------- not: The news are coming on
Unlike English again, nouns f-g are plural in German, i.e. used with verb in plural
f) Die Möbel sind (not: ist) schön ----> literally: The furnitures are nice
g) Die Informationen(*) sind hilfreich --> literally: The information are helpful
h) Die Nachrichten(*) kommen ---------> literally: The News are coming on.
*German uses different ways of forming plural, one of the most common ones is adding -en.
I hope I could help you a bit
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