The past tense of the verb forbid is forbade.
To understand why this is the case, it’s important to look at how irregular verbs function in the English language. Unlike regular verbs that follow a simple pattern of adding -ed to form the past tense, irregular verbs like forbid have unique forms. When you use forbid to indicate that someone was not allowed to do something in the past, you would say they were forbidden or that you forbade them to do it. For example, you might say, ‘The teacher forbade the students from leaving the classroom.’