This is one of the most common English mistakes in Hong Kong. As a grammar lesson it is simple. When should you use is or are? (subject verb agreement) However this problem causes many mistakes in the English of Hong Kong people. The real problem is something else more complicated: what are you counting?
The simple grammar rule for is and are:
Use is when the subject of the sentence is one thing (singular).
Use are when the subject of the sentence is more than one thing (plural).
He is tall.
They are tall.
The real problem:
It is not always clear if something is one or more things. Uncountable things are treated as if they are one thing.
Love is blind.
Bread is healthy.
(Pieces, slices and loaves of bread are countable, but bread, like water, is an uncountable material.)
So often the real problem is trying to figure out if a thing is one thing (singular) or more than one (plural). English is not always easy or straightforward in counting things as one or more.
Things that are always more than one
1. Pants, as well as types of pants: jeans, trousers and slacks
2. Some diseases, such as Mumps and Measles, originally measles meant a red spot, people with this disease always have many red spots. The word mumps may have come from mumble, people with the disease mumble a lot.
3. Scissors, originally from the Latin word caedere meaning "to cut", always having two cutting blades and are always counted as more than one. If broken in two then the person has two blades, but they are no longer scissors. This also applies to scissor-like tools: shears, clippers, cutters, secateurs, tin-snips, pliers, tongs, pincers, tweezers, forceps.... It is rare to ever use the singular scissor, such as in the hyphenated word: scissor-like. The singular form is used as an adjective: scissor action, scissor kick. All of these scissor-like tools are used with a pair (of) or some:
Get me some scissors. (one or more)
Get me a pair of scissors. (one tool, with two cutting blades)
A handful of things that are always singular, even with an s on the end
1. Studied topics like physics, politics, ethics, or social studies
This includes mathematics, often shortened to maths (But is inconsistently math and mathematics in US English)
2. Foods that are more than one of the thing, but are one dish: eggs, beans....
3. Measurements: uncountable things are counted by units of measurement.
Two glasses of beer is just enough for me.
At the dai pai dong, the cost of lunch is only twenty dollars. 1
Only a fifth of the city is angry. 2
1. Money is an uncountable concept, but dollars are countable and “cost” is singular.
2. There is one city, but if you count people there are more than one: Only a fifth of the people are angry.
4. Names of books, movies and so on are always one, even if the actual word, such as Wars, is plural.
Star Wars is a famous movie.
5. Then there’s the whole, vexed question of collective nouns. Some unreasonably strict English teachers, especially in the US, seem to insist collective nouns always take a singular verb, but actually it’s flexible.
Our team is winning.
Our team are wearing red.
Usually when you confuse is or are the sentence will only look or sound awkward. It is unusual for the meaning to be confused. It is much better to try, and be awkward, than never to try. People who try will fail sometimes, but those who never try will never succeed. For minor grammar points, when you are not sure, guess. Real English is not always clear or strict!