"Variable nouns in English have both a plural and a singular form which differ from each other, most often by the addition or subtraction of an 's', though there are, of course, many irregular variable nouns. Invariable nouns in English come in different forms. Some have only a singular form, as is the case with uncountable nouns like furniture and the names of some academic subjects, games and diseases, e.g. mathematics, darts and measles, which look misleadingly like plurals. Others have only a plural form in English, whereas their counterparts in other languages may have singular forms. For example, many objects which are made up of two hinged or joined parts and are symmetrical, such as scissors, scales, secateurs and trousers, have only a plural form in English. To refer to these nouns in the singular, you have to use 'a pair of' or 'some'. If the learner assumes that the number of these nouns is the same in English as in their mother tongue invalid noun forms, like trouser, and incorrect verb-noun and determiner-noun agreement errors, as in 'this trouser is too small' will result. This is all the more confusing since in languages where nouns of this type have been borrowed from English, they are usually given a singular form, regardless of their status in the language they were borrowed from. Similarly, some nouns in English, like sheep, aircraft and offspring have the same form in both the singular and the plural, with only context to help with their interpretation. The learner who has not sufficiently learnt these facts may rely on the assumption that these nouns behave in the same way as they do in their mother tongue."
Source :MED Magazine
Interested in Language