So the passage is talking about soil erosion, mudslides, and the havoc that these two things can cause for people. This would be part of the main idea of the passage. "Homes and other buildings can be destroyed" when people build on steep hills and those hills suffer from mudslides.
Here are the key details of the passage now that you understand the main idea. The hills have clay soils. When these hills are wet and slippery, "the soil on the hills can move downward". The passage then describes what happens when this occurs. People build "retaining walls" to stop the mudslides, but it does not always work.
Hint: The main idea should be brief and to the point. It should be what the passage is mainly talking about. The key details in a passage would be what specifically the author is stating within it. It should still be a brief explanation, but try to be specific. Also, use some quotes from the passage to explain your answer more clearly.
So the passage is talking about soil erosion, mudslides, and the havoc that these two things can cause for people. This would be part of the main idea of the passage. "Homes and other buildings can be destroyed" when people build on steep hills and those hills suffer from mudslides
