Теорія:

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 to a plural one, you should add the ending -s.
 
Shop - shops
Week - weeks
Flower - flowers
Cat - cats
  
1. If a nouns ends with -s, -ss, -sh, -ch, -o, -x , you must add -es:

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 into -ves:

Shelf - shelves
Knife - knives