Many of the aircraft are under repair

Some English words are spelt the same, be they singular or plural. Examples in this group include deer, fish, sheep, and aircraft.

Let us look the the examples of using them in sentences.
1 a) I bought a fish at the pet shop. [singular]
b) There are two fish in the glass bowl. [plural]

2 a) I saw a deer when I went to the zoo. [singular]
b) Three deer can be seen under the tree. [plural]

3 a) There is a sheep near the tree. [singular]
b) The farmer rears ten sheep for fur and milk. [plural]

4 a) Many of the aircraft are under repair. [plural]
b) He bought an aircraft for personal use. [singular]