There are words in the English language that seems like a plural but are to be used as a singular.
Spotting a 's' at the end of a noun is one of the simplest ways of determining its plural form. If only the English language was that simple. There are few nouns that exist only in its plural form. Singularly, it is known as pluralia tantum' which is Latin for 'plural only'. Here are some of them.
Scissors
There is an internal logic of why scissors always maintain their plural form. Some similar ones are tongs, forceps, tweezers, pliers, etc. As you can see, they are either tools or instruments. But that alone doesn't suffice. Because then, trap, clamp, flat iron should not be existing in their bachelor states.
While the last given examples of tools/instruments have two joined parts, the items in contention have two separate ones. Both of these parts must function in unison as a pair for the instrument or tool to work. Hence, we say, “hand me the pair of scissors”. In keeping with the plural verb agreement, we also say, “those scissors are more suitable” and not “that scissor is more suitable.”
Glasses
Extending the above logic, we always mention glasses, spectacles, goggles and binoculars as plurals. But there is a slight modification. Both the parts of the given examples are connected, but both parts have to remain so connected to contribute equally to the functionality of the object. Hence, they are conceived as a single unit. If you may note, a telescope which constitutes two lenses but parallel to each other can take a singular form. Binoculars, on the other hand, are always plural.
Trousers
By definition, trousers, and its variants –pantaloons, pants, jeans, culottes, jodhpurs, etc. are spelt as a plural but reckoned as singular. So, does its near and far relatives like shorts, tights, leggings, etc.
Fashion related reports sometimes do isolate these wearables as in, “a beige pant is a must-have for this summer season.” But it's a one-off usage if you catch my drift.
Rather, even various styles associated with lower-limb fashion are plurals like bell-bottoms, capris, skinnies, etc.
Clothes
Shirts, blouses, trousers, skirts, vests, camisoles, socks, jackets may not exactly be similar if one accounts for them individually, but there is a connection. They can be lumped under the umbrella of clothing items. Thus, we have clothes and never clothe. Don't confuse it with a cloth.
By the same explanation, ways of behaving, greeting, eating, talking, presenting, etc. are collectively bunched as manners.
