Here's why:
* Past continuous describes an action that was ongoing in the past at a specific time. It uses the structure "was/were + verb + -ing".
* "Sing" is a regular verb, so we simply add "-ing" to form the present participle.
Therefore, the past continuous tense of "sing" is "was singing" or "were singing", depending on the subject.
Examples:
* She was singing in the shower.
* We were singing Christmas carols.