**Rule**: The **Simple Past Tense** (negative) is used for a past inaction that caused a subsequent past feeling. **Correct Usage**: I 'was' disappointed (past state) because he 'didn't keep' his promise (the reason, a past inaction). 'hadn't kept' is also correct. **Incorrect Options**: (b) 'wasn't keeping' is incorrect. (c) 'hadn't kept' (Past Perfect) is also a strong correct answer, emphasizing the prior failure. (d) 'doesn't keep' is a present tense.

**Rule**: The **Simple Past Passive** (was/were + past participle) is used for a completed action in the past at a specific time, where the subject is the receiver of the action. **Correct Usage**: The castle is the receiver of the building action. 'was built' is the correct passive form for this completed historical event. **Incorrect Options**: (b) 'had been built' would be used if it was built before another past event. (c) 'built' is active voice. (d) 'builds' is a present tense.

**Rule**: The **Past Perfect Tense** is used for a past action that began before another past action. The adverb 'already' reinforces this. **Correct Usage**: The film's start ('had already started') happened before our arrival ('got to the cinema'). **Incorrect Options**: (a) 'already started' (Simple Past) is common in informal speech but Past Perfect is more grammatically precise. (c) 'was already starting' implies it started at the same moment. (d) 'already starts' is a present tense.

**Rule**: The **Past Continuous Tense** is used to describe a process that was happening at the same time as another event. **Correct Usage**: The process of the sun going down ('was setting') was happening simultaneously with the sky changing color ('turned'). It describes a gradual scene. **Incorrect Options**: (a) 'set' (Simple Past) is also possible. (c) 'had set' would mean the sun was already down. (d) 'sets' is a present tense.

**Rule**: The **Simple Past Tense** is used for a single, completed event at a specific time in the past. **Correct Usage**: 'a decade ago' is a definite past time, so 'collapsed' is the correct verb for this finished event. **Incorrect Options**: (a) 'was collapsing' is a process. (b) 'had collapsed' would need another past reference. (d) 'collapses' is a present tense.

**Rule**: The **Past Continuous Tense** is used for a longer background action that was in progress when it was interrupted by a shorter, sudden action. **Correct Usage**: He was engaged in the ongoing action of riding his bike ('was riding') when the sudden interruption occurred (a car 'turned'). **Incorrect Options**: (a) 'rode' (Simple Past) would suggest the actions happened one after another. (c) 'had ridden' and (d) 'had been riding' are incorrect for this structure of interruption.

**Rule**: The **Past Perfect Continuous Tense** is used to emphasize the duration of a preparatory activity that occurred before a concluding past event. **Correct Usage**: 'had been studying' emphasizes the long, continuous investigation ('for weeks') that preceded the final solution ('solved'). **Incorrect Options**: (a) 'studied' is less descriptive. (b) 'was studying' is the wrong sequence. (d) 'had studied' (Past Perfect) focuses on completion.

**Rule**: The **Past Continuous Tense** is used to describe two or more simultaneous, ongoing background actions that set a scene in the past. **Correct Usage**: 'was shining' and 'were singing' were parallel, continuous activities creating a pleasant atmosphere. **Incorrect Options**: (a) 'shone' (Simple Past) is also possible but less descriptive of the ongoing scene. (b) 'had shone' and (d) 'had been shining' would place the shining before the singing.

**Rule**: This question requires a verb that shows a prolonged, unsuccessful effort before a concluding event. 'trying to sell' captures this. The **Past Perfect Continuous Tense** is used for the duration of this effort before the final event. **Correct Usage**: 'had been trying to sell' correctly describes the long, continuous effort over six months that occurred before they 'received' the offer. **Incorrect Options**: (a), (b), and (d) use the wrong verb ('sell' instead of 'try to sell') or tense.

**Rule**: In Reported Speech with a past reporting verb ('announced'), the verb tense shifts back. A statement in the Present Perfect ('we have had') or Simple Past ('we had') becomes **Past Perfect**. The past perfect of 'have' is 'had had'. **Correct Usage**: The profitable year ('had had') occurred before the company 'announced' it. **Incorrect Options**: (a) 'had' (Simple Past) is also possible. (b) 'was having' is an incorrect shift. (d) 'has had' is a present tense.