Yes, the word "air" is a noun or a verb.
The noun air is a word for the atmosphere, or the "air" or feel of a location.
The verb "to air" means to present or show, or to place in the air for cooling, freshening, or cleaning.
Singular
Singular Plural is caterpillars
The word crisis is singular. The plural form of crisis is crises.
Steroids is plural. The singular form of the word is steroid.
Association is singular. Associations is plural.
The word team is singular; the plural form is teams.
These is plural, this is singular
Lance is singular. Lances is plural.
The word 'Phenomenon' is singular. The plural form is 'Phenomena'.
Singular
The word metastasis is singular, metastases is plural.
The word "ground" can be both singular and plural. Singular: "The ground is wet." Plural: "The grounds of the park are well-maintained."
Louse is singular. The plural form is lice.
One alveolus is a singular, tiny air sac of the lung, surrounded by a network of capillaries, and through which oxygen and carbon dioxide are exchanged. A cluster of such air sacs are alveoli (plural of alveolus).
No, the word "list" is singular. The plural form of "list" is "lists."
It's singular. The plural is cafeterias.
Verbs cannot be singular or plural. Has is after singular nouns.