Complex sentence: Economists generally believe that deflation is a problem in a modern economy because it may increase the real value of debt, especially if unpredictable, and may aggravate recessions and lead to a deflationary spiral.
Split into simpler sentences: Economists generally believe that deflation is a problem in a modern economy because it may increase the real value of debt, especially if the deflation was unexpected. Deflation may also aggravate recessions and lead to a deflationary spiral.