The Past Continuous tense describes actions that were ongoing in the past. It is formed using the past tense of the verb "to be" (was/were) followed by the present participle (verb + -ing). For example, in the sentence "I was reading a book," the action of reading was happening at a specific time in the past. This tense is often used to set the scene in storytelling or to indicate that one action was interrupted by another. For instance, "While I was walking in the park, it started to rain." Here, the walking was ongoing when the rain began.