The document discusses the past continuous tense in English grammar. The past continuous tense is used to describe actions that were ongoing or in progress at a specific time in the past. It is formed using the past tense of the verb "to be" (was/were) plus the present participle form of the main verb ending in "-ing". Examples are provided such as "I was doing my homework when you came in" and "They were taking the dog for a walk when it started to rain".