[答案]
Icouldnot/couldn'tplaybadmintonfiveyearsago.
[解析]
考查肯定句转否定句。I could play badminton five years ago.(五年前我就可以打羽毛球了。)时态为一般过去时的陈述句。改为否定句需把could改为could not或couldn't。
故答案为:I could not /couldn't play badminton five years ago.
[点评]
本题考查了"肯定句与否定句转换,",属于"基础题",熟悉题型是解题的关键。