1) 文章として複数にするのか、単数にするのか判断（一般化した話でもニュアンスが異なる）
A motorbike is cheaper than a car.
American people are bigger than Japanese people.

2) 特殊な単語ではないか、物質名詞ではないか（集合名詞、抽象名詞も複数形にはならないが間違いにくいので）
sがないけど複数形：people, police, audience
ふつう数えられないので単数形→
-意外な物質名詞（数えられないので単数、個々種類を意識するときは複数になる場合も：food, fruit, fish, meat, chicken
-性質を表すときは物質名詞なので単数：paper, wood
-集合名詞：furniture, baggage, advice (個々の家具の総称)

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■every, each
everyやeachの後ろの名詞は単数　every student is required to study that.
each of the複数形で複数を取れ単数扱い　Each of the movies sounds interesting.
複数名詞+eachで複数扱い　They each have their own room.

■No, either, neither
no, either、neitherは常に単数扱い　Either Jane or Mary has to take care of it.

■ALL
allは複数　All the participants of the project were present at the meeting.
all+不可算は単数　All of the money is spent.
All studentsは全世界の生徒全員、All the studentsはある範囲の生徒

■and
通常は複数
Their application form and resume are essential if they will go to the job fair.
1セットで考えられる場合や同じ対象をさす場合は単数
Coat and tie is the attire required for men who are getting the interview.

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不可算名詞にはa little、可算名詞にはa few　I had only a little experience in that.