How do you know to use was or were in a sentence?

Answer 1

As the past tenses of am and are, was is singular and were plural.

• I was jogging.
• We were jogging.

In sentences of the form "If ____ was/were , .", use "was" when you don't know whether or not is true, in which case is applicable only when is true. Use "were" when you know that is false (did not happen), in which case refers to what would have been if were true.

• If Mr. Miller was here yesterday, I need to get his homework assignment.
• If Mr. Miller were here yesterday, he would have assigned homework.
• If I was President, I must have blocked it out, because I don't remember being President.
• If I were President, I would do so many things differently.

Answer 2

Use "was" when referring to singular subjects and "were" when referring to plural subjects or when using the subjunctive mood. For example, "He was at the store" (singular) versus "They were at the store" (plural).