**Rule**: The **Simple Past Tense** is used to describe a state that was true in a completed past period. **Correct Usage**: 'in the 1980s' is a finished past period, so 'was' correctly describes the state of the city at that time. **Incorrect Options**: (b) 'had been' would be used if this state existed before another past event. (c) 'was being' is for temporary behavior. (d) 'is' is a present tense.

**Rule**: The **Simple Past Tense** is used for a completed action that set the stage for a longer, ongoing action. **Correct Usage**: First, the police 'cordoned' off the area (a completed action). Then, inside that area, they 'were investigating' (an ongoing action). **Incorrect Options**: (b) 'were cordoning' is an ongoing process. (c) 'had cordoned' would mean they cordoned it off long before the investigation. (d) 'cordon' is a present tense.

**Rule**: The **Simple Past Tense** is used to describe a past state that was the reason for another past state. The verb 'have' for possession is stative. **Correct Usage**: The company 'was' successful (past state) because the founders 'had' a clear vision (the reason, also a past state). **Incorrect Options**: (b) 'were having' is incorrect as 'have' is stative. (c) 'had had' (Past Perfect) would imply they had the vision long before the company was successful. (d) 'have' is a present tense.

**Rule**: The **Past Continuous Tense** is used to describe a background weather condition that was happening during another past activity. **Correct Usage**: The snow 'was falling' (the ongoing background condition) at the same time that 'we drove'. **Incorrect Options**: (a) 'fell' (Simple Past) is a completed action. (c) 'had fallen' means the snow was already on the ground. (d) 'had been falling' would emphasize the duration before the drive.

**Rule**: The **Past Perfect Tense** is used for a past action that was completed before another past action occurred. **Correct Usage**: They 'had finished' dinner; the meal was completely over before the doorbell 'rang'. **Incorrect Options**: (a) 'finished' is less precise. (b) 'were finishing' means they were at the very end when the bell rang. (d) 'had been finishing' is not a standard construction.

**Rule**: The **Past Perfect Tense** is used for a recent past event that was the cause of a subsequent past situation. **Correct Usage**: The hall 'was' packed (past state) because the band 'had just released' a new album at a time immediately prior to that. **Incorrect Options**: (a) 'released' (Simple Past) is also possible. (b) 'was releasing' is illogical. (d) 'releases' is a present tense.

**Rule**: The **Past Perfect Continuous Tense** is used to emphasize the duration of an ongoing action that was interrupted by a sudden past event. **Correct Usage**: 'had been speaking' emphasizes the continuous speech ('for about an hour') that was in progress before the microphone 'stopped working'. **Incorrect Options**: (a) 'spoke' is less descriptive. (b) 'was speaking' is also possible, but less emphatic about the preceding duration. (c) 'had spoken' (Past Perfect) focuses on completion.

**Rule**: The **Past Continuous Tense** is used for an ongoing action that was interrupted by a short, sudden action. The phrase 'just as' often signals this. **Correct Usage**: The speaker was in the process of leaving ('was leaving') when the phone 'rang'. **Incorrect Options**: (a) 'left' (Simple Past) suggests the speaker had already gone out. (c) 'had left' means the speaker was already gone. (d) 'leave' is a present tense.

**Rule**: The **Past Perfect Tense** is used for a past event that caused a subsequent past emotional state. **Correct Usage**: She 'was' crying (past state) because the event of failing her exam ('had failed') had happened before that. **Incorrect Options**: (a) 'failed' (Simple Past) is also possible. (b) 'was failing' is an ongoing process. (d) 'fails' is a present tense.

**Rule**: The **Past Perfect Tense** is used to talk about a past state or repeated action that occurred before another key event in the past. The word 'Before' often signals this. **Correct Usage**: His work as a waiter ('had worked') took place in the period before he 'became' a famous actor. 'had been working' is also correct and emphasizes the duration. **Incorrect Options**: (a) 'worked' (Simple Past) is also possible, but Past Perfect is more precise. (b) 'was working' is less appropriate for a state over 'many years'. (d) 'had been working' is also a strong correct option.