Using Was & Were 

We use "was" when the subject/noun is singular (I, he, she, it, or a singular noun) and you are describing something that happened in the past. 

Example 1: I was at the park yesterday. 

Example 2: The dog was playful.

We use "were" when the subject is plural/noun (we, you, they, or a plural noun) and you are describing something that happened in the past. 

Example 1: We were at the concert last night. 

Example 2: The students were studying.

We also use "were" when the subject is talking about something imaginary. Even if the noun is singular. 

For example: If I were rich, I would not have to go to school.