Singular-only nouns (also called mass nouns) refer to nouns that are either abstract or too numerous to count (because they are a substance or exist in masses).
Examples include concepts or traits like knowledge, justice and wisdom. Other examples are substances that are non-quantifiable, or exist in a form which cannot be counted, (e.g., milk, rice or water).
Mass nouns, also non-count nouns, are things or concepts that we cannot physically count. Think fire, sand and water, which are things we cannot count.
The plural and singular of series is series. Series already refers to a number of things, and so it stays the same either way.
Aircraft is the same in both its singular and plural noun form. Both are aircraft.
Offspring is a mass noun with no plural form.
Swine is used for both the singular and plural forms of swine (as in the ruminant, the pig).
As a general guide, beer (no -s) is most often the plural of beer. When referring to varieties, or types of beers, the standard pl. n. is acceptable.
Species is plural for species. Species is already a plural noun.
As a course of action, means can be singular and plural. When referring to finance, means is always plural.
Software is a mass noun that only has a singular noun form. Whether we use it as a singular or plural, it stays “software”.