Теория:

Study the exceptions with plural and singular nouns.
 
These things are always plural in English:
  
Scissors
Glasses
Trousers
Jeans
Shorts
Tights
Pyjamas
  
Do you like jeans?
  
feet-349687_640.png
Рис. \(1\). Jeans

Do you wear glasses?
  
glasses-1246611_640.png
Рис. \(2\). Glasses
 
But you can say a pair of trousers/pyjamas/etc. Some plural do not end with -s.
  
Remember these nouns.
 
Singular Plural
ManMen
WomanWomen
ChildChildren
FootFeet
ToothTeeth
MouseMice
SheepSheep
FishFish
GooseGeese
DeerDeer
OxOxen
PersonPeople
CrisisCrises
BasisBases
AnalysisAnalyses
PhenomenonPhenomena
FormulaFormulae
 
People -  plural
Police - plural
 
bodyworn-794111_640.png
Рис. \(3\). Police
 
1. The police are here.
2. People are very calm and friendly there.
Источники:
Рис. 1. Jeans, 2011, https://pixabay.com/images/id-349687/ Pixabay License CC0 Free for commercial use No attribution required
Рис. 2. Glasses, 2016, https://pixabay.com/images/id-1246611/ Pixabay License CC0 Free for commercial use No attribution required
Рис. 3. Police, 2015, https://pixabay.com/images/id-794111/ Pixabay License CC0 Free for commercial use No attribution required