**Rule**: The **Past Perfect Tense** is used to describe a state that was true (or not true) before a specific event in the past changed it. **Correct Usage**: The state of not having seen snow ('had never seen') existed for her entire life up until the point she 'moved' to Canada. **Incorrect Options**: (a) 'didn't see' and (d) 'never saw' (Simple Past) are also possible, but Past Perfect is more precise about the experience *before* that point. (b) 'wasn't seeing' is incorrect.

**Rule**: This sentence shows a past state caused by a long, continuous prior action. The state is described with the **Simple Past**, and the causal action is described with the **Past Perfect Continuous**. **Correct Usage**: He 'was' exhausted (past state) because he 'had been driving' for a long duration before that moment. **Incorrect Options**: The other pairings create an illogical sequence of tenses. (a) 'was, drove' doesn't show the cause. (b) 'was, was driving' implies the actions were simultaneous. (d) 'had been, drove' is an incorrect combination.

**Rule**: **'Could'/'Couldn't' + base verb** is the past form used to talk about a general ability or inability in the past. **Correct Usage**: 'couldn't find' correctly describes the past state of inability that prompted the action 'had to retrace'. **Incorrect Options**: (b) 'wasn't finding' is awkward. (c) 'hadn't found' would be used if this state existed before another past action. (d) 'don't find' is a present tense.

**Rule**: The **Simple Past Tense** is used for a completed action at a specific time in the past. 'was trying' would also be correct, emphasizing the repeated effort. **Correct Usage**: 'tried' simply states the completed action that took place 'yesterday'. **Incorrect Options**: (b) 'was trying' (Past Continuous) is also a very good option, as it can show a repeated, frustrated action in the past. (c) 'had tried' and (d) 'had been trying' would be used if this action happened before another past action.

**Rule**: The **Past Perfect Tense** is used to describe the duration of a state leading up to a specific past event. **Correct Usage**: The state of being married ('had been') existed for 50 years up to the point that they 'celebrated' their anniversary. **Incorrect Options**: (a) 'were' (Simple Past) is less precise. (c) 'were being' is incorrect. (d) 'have been' is a present tense.

**Rule**: The **Simple Past Tense** is used to narrate a sequence of completed past events. **Correct Usage**: First, the meeting 'concluded'. Second, everyone 'agreed'. This is a clear narrative of two sequential past actions. **Incorrect Options**: (b) 'was concluding' is a process. (c) 'had concluded' would be used if the agreement happened much later. (d) 'concludes' is a present tense.

**Rule**: The **Simple Past Tense** is used for a short, sudden action that happens during a longer, ongoing action (which is in the Past Continuous). **Correct Usage**: The breaking of the leg ('broke') was a sudden event that occurred during the longer activity of 'was playing'. **Incorrect Options**: (b) 'was breaking' suggests a slow process. (c) 'had broken' would be used if this happened before another past event. (d) 'breaks' is a present tense.

**Rule**: The **Simple Past Tense** is used for a completed action that happened at a specific time in the past, especially for historical events. **Correct Usage**: 'in 1928' specifies a definite past time, so 'discovered' is the correct choice. **Incorrect Options**: (b) 'was discovering' is an ongoing process. (c) 'had discovered' would need a subsequent past reference. (d) 'discovers' is a present tense.

**Rule**: The **Past Perfect Tense** is used for a past event that occurred before and caused a subsequent past state. **Correct Usage**: The castle 'was' dark (past state) because the power cut ('had happened') occurred prior to that time. **Incorrect Options**: (a) 'happened' (Simple Past) is also possible. (b) 'was happening' implies the cut was in progress. (d) 'happens' is a present tense.

**Rule**: The **Simple Past Tense** is used for a past state that provided the reason for a past conclusion. 'Add up' here is an idiom meaning 'make sense'. **Correct Usage**: The detective 'knew' (past state) because the story 'didn't add up' (the reason, also a past state). **Incorrect Options**: (b) 'wasn't adding up' is also possible, emphasizing the ongoing inconsistency. (c) 'hadn't added up' would be used if the story was told before the detective's realization. (d) 'doesn't add up' is a present tense.