"Was" or "Were" are the past tenses of the verb "To Be" and it's important to be careful when choosing the correct verb for the subject of a sentence.
You should select the correct verb form from the following:
They were *
"You were supposed to clean your room today."
"He was supposed to clean his room today."
"I was supposed to clean my room today."
"They were supposed to clean their rooms today."
"She wasn't supposed to arrive so early."
"The sheep were grazing in the field." (many sheep)
"The early 1940s was a period of war. / The early 1940s were years of war."
*     "They were" can also be used for gender-neutral singular cases when the person doesn't identify as a he or she.
So why do we sometimes hear people say "I were", "he were" instead of "I was", "he was"? This is because some verbs have moods and this is the past subjunctive form/mood of the verb "To Be". It is used in hypothetical/fantasy scenarios that are unlikely to happen.
Look at the following examples:
If I were more confident, I would sing.
I wish she weren't so late all the time!
If only I weren't so forgetful!
I wish I weren't so lazy!
I wish it were summer.
He behaves as if he were the owner.
In each of these examples above, the use of the word "if" or "wish" is the clue that the past subjunctive form should be used.
Complete the sentences with was / were - Answers below.
1. "Your parents _______ neighbours of mine many years ago."
2. "There _______ a dozen candidates for the job."
3. "If he _______ a better player, he could make the team."
4. "When I _______ younger I wanted to be a footballer."
5. "________ there many people at the concert?"
1. were, 2. were, 3. were, 4. was, 5. were