Investors & landlords

It is a 4 unit house and the unit that had the repairs was rented at the time and there was no displacement of the renter. 

 

My understanding of deductible expenses was that the tax year that you pay for the expense is when you deduct it. So I wouldn't be able to deduct something I hadn't paid for yet or deduct something that I had paid for in the previous year. Unless I'm wrong on this?