Rewrite each pair of sentences by replacing relative clauses with " to v"

Ex7. Rewrite each pair of sentences by replacing relative clauses with TO V-INF
1. Ethan is usually the last person who understands the joke.
=> ___
2. His office was the first room that they had to clean.
=> ___
3. We had a river in which we could swim.
=> ___
4. Here is the scientific paper that you must read.
=> ___