Final answer:
The two sentences differ in that the first uses past perfect passive voice with a focus on an object, while the second is in simple past active voice with a focus on an action during an event.
Step-by-step explanation:
Two grammatical differences between the sentences 'It had a long strap, and she carried it slung across her shoulder.' and 'We took a road trip, and my sister complained from the back seat the whole time.' can be discerned from a grammatical standpoint. The first sentence uses the past perfect tense to describe the ongoing state of carrying an object, indicated by the word 'slung,' which is a past participle used in passive voice. In contrast, the second sentence is in simple past tense and has an active voice, as shown by the verb 'complained.'
Furthermore, the structure of the two sentences reveals another difference. The first sentence's structure emphasizes the possession and action toward an object by using a pronoun 'it' to refer to the object with the long strap, followed by a description of how it was carried. Conversely, the second sentence centers around the action ('complained') and the circumstances under which it occurred ('from the back seat the whole time'), focusing more on the activity during the road trip rather than an object or possession.