The key to understanding when to use was or were in a sentence is determining whether you need to use the subjunctive mood or not. A verb is in the subjunctive mood if it expresses an action or state that is not reality. For example, it might be hypothetical, wished for, or conditional.
To better see what we are up against when deciding when to use was or were, let’s compare the past and subjunctive conjugations of to be side by side.
If you find yourself debating whether to use “was” or “were” in a sentence, it’s likely that you’re dealing with an unreal conditional sentence. As a refresher, an unreal conditional sentence expresses events that are hypothetical or improbable.
Typically, an unreal conditional sentence begins with an if clause containing the past tense or past perfect tense of a verb followed by a conditional clause containing a modal verb such as “would.” Consider the following sentences:
You only have to observe a dog rooting around in the trash can to realize that dogs don’t understand what could happen if they eat spoiled food. Humans, on the other hand, have the power of reason. Rotten meat is dangerous. They can imagine various possible consequences—a tummyache, a trip to the hospital, expensive prescriptions, and so on. Conditional sentences reflect humans’ capacity to hypothesize.
Many writers wonder if it’s equally correct to use “will” or “would” in an if-clause. The short answer is no, but there are exceptions to the rule. An if- or when-clause (often used to form conditional sentences) generally does not contain “will,” which is the simple future tense of the verb “to be.” One exception is when the action in the if- or when-clause takes place after that in the main clause.
