Theorie:
Study the information about plural and singular nouns.
The difference between singular and plural nouns is easy to see. When a noun indicates one only, it is a singular noun. When a noun indicates more than one, it is plural. If you want to turn a singular noun into a plural one, you should add the ending -s.
Shop - shops
Week - weeks
Flower - flowers
Cat - cats
Week - weeks
Flower - flowers
Cat - cats
Fox - foxes
Bus - buses
Potato - potatoes
Exception: photo - photos
2. If a noun ends with -y, you should drop -y and add -ies:
Party - parties
Baby - babies
But! With oy, uy, ay, ey you must add only -s:
Monkey - monkeys
Boy - boys
3. If a noun ends with -f or -fe, change it into -ves:
Shelf - shelves
Knife - knives