it's years since they love with each other

A.固定句型：“It +be +some time +since…”
“自……开始（结束）以来,已经过去多长时间”.
主句为“It is/has been+时间段”时,since从句通常用一般过去时；
主句为“It was/had been+时间段”时,since从句一般为过去完成时.
It is two months since he fell ill.
It has been over ten days since she left home.
It was over ten days since she had left home.不能选Afall in love 不能和时间状语连用啊 只有be in love 才和事件状语连用这是两个概念。这里表示“自从他们相爱（那一刻，短暂动作）以来，已经好几年了。ten years 是主句中的成分，fell in love 是since 从句中的谓语。明白了吗？祝你进步！