Some English nouns are normally used only in the plural form, even when they name one object. These are often called plural only nouns or pluralia tantum.
Common examples include scissors, trousers, glasses, binoculars, and pants. We usually treat these words as plural because the object has two matching parts, or because English developed a plural form that became the standard name.
For example, we say, These scissors are sharp, not This scissors is sharp. We also say, My trousers are too long. If you want a singular countable phrase, English often uses a pair of: a pair of scissors, a pair of trousers, a pair of glasses.
Compare this with a normal singular noun like shirt. We can say one shirt or two shirts. But with scissors, the usual pattern is one pair of scissors or two pairs of scissors.
- Correct: The glasses are on the table.
- Correct: I bought a new pair of glasses.
- Less natural: a glass, because that means a drinking container, not eyewear.
This is one of those small grammar patterns that makes English feel unusual, but once you notice it, the usage becomes much easier.