The most common way to change a singular noun to a plural one is to simply add an s. There are some basic rules for pluralising nouns.
Changing y to ies:
puppy puppies
lorry lorries
teddy teddies
Changing fe to ves:
knife knives
wolf wolves
leaf leaves
However, not all nouns follow the rules. The plural of roof is roofs, not rooves, but the plural of hoof can be either hoofs or hooves. Confusing, isn't it?
Some nouns ending in o add es in the plural while others just add s:
tomato tomatoes
radio radios
There are words that don't change at all in the plural:
sheep sheep
fish fish
There are also words that change completely in the plural:
person people
mouse mice
child children
Sometimes we just have to learn the plurals, although a dictionary can be very helpful.
Let's move on to the questions now.