Q: Is the behind leg supposed to be straight in Tai Chi?
A: No, only if you practice Wu style (Wu Jianquan), or the Wudang style of Cheng Tin Hung (Zheng Tianxiong) 鄭天熊 which is based on Wu style. Wu Jianquan was an accomplished martial arts master. He practiced and taught the postures leaning forward with the behind leg stretched. But, as illustrated, not completely straight.