There are nouns in Polish that appear only in plural form despite having a singular meaning. Because of that, in case of such type of words, even if we will refer to one object we will use grammatical construction reserved typically for plural nouns.
Group of Pluralia Tantum is rather small in Polish. Take a look at most common ones organized in groups which may help you memorize them.
Objects that consist of few parts, usually set in symmetric way (mirror reflection when divided on half)
Names of special periods and dates
Wakacje (summer holidays)
Ferie (winter holidays)
Imieniny (name day)
Some geographic names
Tatry (Tatra Mountains)
Czechy (Czech Republic)
Others: plecy (back), skrzypce (violin), perfumy (perfume)