Final answer:
In the provided sentence, the singular nouns are 'bird' and 'tree'. The plural forms would be 'birds' and 'trees', making the sentence 'The birds were flying over the trees'. This process is part of basic English grammar, but keep in mind there are exceptions for some nouns.
Step-by-step explanation:
In the sentence, 'The bird was flying over the tree,' both 'bird' and 'tree' are singular nouns. To make these nouns plural, we usually add an 's' at the end of the word. So, 'bird' becomes 'birds' and 'tree' becomes 'trees'. Hence, the sentence changes to 'The birds were flying over the trees.'
This is a basic rule in English grammar for changing singular nouns into plural. However, remember that some nouns undergo different changes or irregular transformations when they become plural.
Learn more about Plural Nouns