Brianna: It would have been better to buy a tree last summer rather than this summer.
Evidence
The one we bought this summer is struggling to survive this summer’s drought.
Last summer’s normal rainfall would have enabled it to develop established roots.
Trees with established roots can better withstand droughts.
Intermediate Conclusion
If we had bought one last summer, it would have been able to survive this summer’s drought.
Main Conclusion
It would have been better to buy a tree last summer rather than this summer.