Noun phrases: noun phrases and verbs The head of a noun phrase Nouns singularity and plurality

A Swedish perspective: Inherent plurals (click to expand/contract)

Nouns in -ics

Nouns that end in -ics look plural, but are actually most often treated as singular. Thus, when heading a noun phrase which functions as the subject, they trigger singular agreement on the verb.
(16) Statistics is becoming increasingly popular among our students.
(17) Mathematics is an integral part of our culture.
(18) Western economics has tended not to be influenced by theories from other parts of the world.
In the examples above, the nouns in -ics denote academic disciplines. However, some of these nouns may also be used to denote the practical application of the discipline, and are then treated as ordinary plurals, e.g. by taking plural determiners and by triggering plural agreement on the verb.
(19) These statistics show that our production of beef has almost doubled.
(20) The acoustics of the new concert hall are very lively.
Zero plurals
Zero plural nouns are nouns that look the same in the plural as they do in the singular. A well-known example is the noun sheep. Since sheep is a zero plural noun, it looks the same in the two sentences below. However, this does not prevent it from being singular in the first sentence and plural in the second one, as indicated by the different verb forms, is and are:
(21) My sheep is black.
(22) My sheep are black.
Other nouns that belong to this category are aircraft, Chinese, deer, elk, headquarters, horsepower, hovercraft, means, offspring, Portuguese, salmon, series, species, trout, and Vietnamese. When in doubt, please consult a good dictionary.
Foreign plurals
There is a group of nouns whose members are commonly referred to as 'foreign plurals'. What the nouns in this group have in common is that both their singular and their plural forms have been borrowed from other languages, which means that the plural ending is not the regular English -s, but something else.
Examples of such foreign plural nouns that are important to remember, especially when writing academic texts (since many of these words tend to be academic in nature), are analysis-analyses, basis-bases, criterion-criteria, diagnosis-diagnoses, hypothesis-hypotheses, parenthesis-parentheses, phenomenon-phenomena, stimulus-stimuli, and thesis-theses. 
What usually happens when a word is borrowed into English (or into some other language) is that it is changed in line with the morphology of the language into which it has been borrowed. Consequently, there are some foreign words in English that have both a foreign and an English plural form. Examples include appendix-appendixes/appendices, cactus-cactuses/cacti, focus-focuses/foci, and index-indexes/indices.