it was after the failure of this attempt that he resorted to force.这是对 状语 强调的 强调句型
= he resorted to force after the failure of this attempt
所以 里的that he resorted to force不是真主语
has to do had to do和has had to do的区别 是时态上的区别1 一般现在时2 一般过去时 3 现在完成时
the shoulders and the back which is还是are caused by pressure and tension in work
都对
前者 先行词 是 the back
后者先行词 是 the shoulders and the back
谓语单复数是 根据先行词 而定
可根据 句子意思 判断