Final answer:
The sentences should be completed with 'should have stayed,' 'should have paid,' 'shouldn't have left,' and 'must have left without noticing' to indicate advisability and past actions.
Step-by-step explanation:
The student's question requires filling in the blanks with the positive or negative form of 'should have,' 'must have,' or 'could have' in combination with the verbs provided. When reconstructing the sentences, the correct forms to use are:
- We should have stayed longer because it was so cold.
- We should have paid more attention to our fire.
- We shouldn't have left without checking that the fire was out.
- We must have left the forest without noticing that the fire was still burning.
These sentences show the use of modal verbs to indicate advisability and assumptions about past actions.