Some nouns have different meanings in the singular and the plural. Examples are given below:

Air – atmosphere
Airs – affected manners

Good – beneficial, suitable, skilled, kin, virtuous etc.
Goods – merchandise, portable property

Respect – state of being admired, esteem
Respects – compliments, regards

Physic – medicine
Physics – natural science

Iron – a useful metal
Irons – fetters

Force – strength
Forces – armed troops

Related posts:

1. Nouns with Identical Singular and Plural Forms
2. Formation of Plural Nouns
3. Collective Nouns and Abstract Nouns
4. Proper Nouns and Common Nouns
5. Countable and Uncountable Nouns
6. Noun: Number