|1||The blonde girl immediately fell asleep. When the bears returned to their cottage, they saw that she had been eating their porridge.|
|2||Cinderella watched the prince ride up to her house. The prince tied it up and he sacked his servants.|
Coherence is to do with the way in which parts of a text work together or make sense in terms of meaning, while cohesion involves the textual linkages betweeen parts of a text, such as using pronouns to refer to something previously mentioned.In the sentence in 1, there are multiple instances of how pronouns are used to refer to previously mentioned entities, and it is understanding these links that helps us to make sense of the text as a whole. So we understand she in the second sentence to refer back to the blonde girl in the first sentence, and we understand their and they in the second sentence to refer to the bears earlier in the same sentence. But we also make sense of the whole text in terms of the meaning that develops, such as recognising that there is a sequence of events - bears go out, girl eats their porridge then goes to sleep, bears return. Of course we are helped in our understanding of this by prior knowledge that we bring to this text, so if we are familiar with the story of Goldilocks and the Three Bears then we will immediately be able to work out what is going on.
The sentences in 2 don't work well together for two reasons, one to do with cohesion and the other to do with coherence. In terms of cohesion, note that there is only one possible antecedents for it, namely house, and it doesn't make sense to tie up a house. Although ride in this context could be understood to involve a horse, this is not explicitly mentioned, and so the anaphor it cannot readily be taken to refer to the prince's horse. Example (12.28) in the textbook is equivalent to this: Keith drove to London. It kept overheating does not work in the same way as (12.27) does: Keith drove to London. The car kept overheating. In terms of coherence, the sentences in 2 don't work because there is nothing in the preceding text that motivates the prince sacking his servants.