触及微妙的问题, 碰到棘手问题
小心翼翼, 如履薄冰

on delicate ground tread the ground tread as on eggs tread on the juice hollow on worn tread tread on eggs tread on air ground on delicate tread on the heels of tread on sb.'s toes on delicate ground to tread on air tread as on eggs tread on sb's toes tread on sb's corns tread on eggs tread on air delicate ground on tread on the heels of tread on sb.'s corns tread on the neck of