Final answer:
The answers for each sentence are: 1) through 2) down 3) under 4) into 5) across
Step-by-step explanation:
1. We walked through the forest in the snow – it was beautiful.
2. The mountain was very high so I didn't look down.
3. It's very hot, so let's sit under those trees.
4. When we went into the cave, it was very dark!
5. Alice took a boat across the lake because she was staying on the other side.