There are four groups of words which some speakers and writers have difficulty with. In each case it has to do with the agreement of plurals or plural-looking words with the verbs or other words they go with.
Some nouns that end in -s look like they are plural, but they really are singular. This is particularly true of branches of knowledge, certain foods or dishes, and certain diseases.
Branches of knowledge like mathematics, physics, ethics, politics, or social studies are singular.
Names of foods, while plural, are treated singularly when they are treated as a single dish.
Some diseases, while plural in origin, are treated singularly because just one disease is discussed: measles, mumps, rickets, or pox.
Examples: Politics is a rough life.
Baked beans is one of my favorite dishes.
Mumps has been nearly eradicated in the U.S.
A few words, though singular in nature, are made of paired items and generally treated as plural: scissors, pants, trousers, glasses, pliers, tongs, tweezers, and the like. Many are often used with the word pair as in pair of pants or pair of scissors.
Example: These scissors are too dull to cut with.
Nouns Expressing Measurement
A noun expressing an amount or measurement is normally singular.
If the unit of measurement refers to a number of individual items, then it treated as a plural.
Examples: Two spoons of sugar is too much for me.
(A single measurement)
Twelve dollars is less than what I want to sell it for.
(A single sum of money)
Four-fifths of the country is satisfied with its health insurance.
(One part of a whole)
Four-fifths of the people are satisfied with their health insurance.
(Four-fifths refers to many individuals.)
Titles of books and other works of art are always considered singular even if the title sounds plural.
The Alfred Hitchcock film The Birds was successfully advertised with a campaign that said, "The Birds is coming!" Unlike so many ads, that one was grammatically correct.
Plurals That Do Not End in -s
A number of plurals, mostly derived from Latin, do not end in -s. Nevertheless, they are plural and should be treated as such. Words such as criteria, phenomena, memoranda, and media are plural. Their singular forms are criterion, phenomenon, memorandum, and medium.
The word data is also technically plural, but the singular form, datum, is rare in English, so using data as singular is tolerated, but not precisely correct. Say "piece of data" or "item of data" for the singular if datum sounds too affected.
See also "Alumni" and other listings in Spelling Slammer.