to fall in love with （someone）；
to fall head over heels in love; to love （someone） from head to toe.
都是指男欢女爱，沉溺情海，或是爱得晕头转向。
例如：
* They have fallen in love with each other for years.（他们相爱多年）。（动词时态：fall, fell, fallen）
* He fell head over heels in love with her.
（= He fell for her head over heels = He is head over-heels in love with her.）（他与她沉溺情海，爱得晕头转向。）多半指很快落入情海。
* He loves her from head to toe.（他从头到脚地爱她）。多指经过较长时间後才热恋, 或
* He falls madly in love with her.
至於男女热恋时「目中无人」或「形影不离」，也可以说:
* They are two peas in a pod.（他们像豆荚里的两粒豆子--即形影不离。）
* They only have eyes for each other（and forget the entire world.）（世界上似乎只有他们两个人。）
来源:恒星英语学习网-口语频道