*There are a few nouns in English in which the plural form is the same as the singular form, e.g.

He is as timid as a sheep.

We are grazing the sheep in the next field.

The speed of this aircraft is very high.

All aircraft must fuel before flight.

The dog roused a deer from the bushes.

Male deer usually have horns on their heads.

My goldfish has died.

The fisher drew in the net and found two goldfish in it.

The fisherman closed the net and found two goldfish in it.

Common singular and compound homonyms of nouns are:

a. the names of certain animals:

bison(bison) deer grouse

cod(cod) fish goldfish

halibut(halibut) mullet(coho)

salmon( salmon)

moose reindeer sheep

shellfish trout whitebait

b.craft and words made up of craft:

aircraft craft(ship) hovercraft(boat) ship) hovercraft(气垫船) spacecraft(宇宙飞船)

c.表示某国人的名词：

I'm proud I am a Chinese.

The Chinese eat with chopsticks.

His wife is a Vietnamese.

The Vietnamese are noted for their cookery.

It was invented by a Swiss.

The Swiss did not take their fate lying down.

Tips: You can't say an English, you have to say an English man, and you have to add -s to some words, like Scot(s).

d. There are some other singular and compound homonyms (italicized parts):

This rare bird has become an endangered species.

There are over 200 species of fish.

...

*There are certain words that have special cases:

Fruit: usually used as a general or collective noun without a plural form:

I like fruit and vegetables.

This pudding has two pounds of fresh fruit in it.

The potato is a vegetable, not a fruit.

The country exports tropical fruits.

They enjoy the fruits of their hard labor.

hair: usually singular, a generic term for hair or animal hair:

She was slender and had long dark hair.

My hair has grown very long.

But it can be pluralized and used as a countable noun when it means a hair or hair:

There are a few hairs in the book.

He had a few gray hairs.

*There are some nouns that have two plural forms, for example:

The herring(s) usually live in large shoals.

Zebra(s) are a more difficult prey.

Some other words with -s endings, usually used as singular (italicized parts):

The accident took place at a crossroads.

The business cycle is a series of peaks and troughs.

The business cycle is a series of peaks and troughs.