1.at a time
at a time[简明英汉词典]
依次,逐一,每次
for a time
for a time[简明英汉词典]
短时间内,暂且
2.by the side在旁边
in the side在一侧
on the side[简明英汉词典]
①〈非正〉作为兼职〔副业〕 ② 暗地里,秘密地
at the side在一边
at the side of与……相比,在……的旁边