A noun is said to be singular if it represents a single or one person, place or thing.
- An idea came into my mind.
- A teacher shapes the future of his/her students.
- The visitor had brought gifts for the children.
- Every child has the right to education.
- The maid looked after the house well.
The highlighted words in the above sentences indicate ‘one’ or ‘single’ nouns. Therefore, they are all singular in number.