for a moment,“为一片刻”,是指片刻、一会儿或一段简短的时间
简单用法：
wait for a moment 等一会儿
come here for a moment 过来一下
for the moment,“为那时刻”,是指那时、那一刻,带有强调、特指的性质.
简单用法：
wait for the moment to come 等那时刻来临
They have their reasons for keeping their marriage a secret for a moment.
他们有自己的原因,暂时将他们俩的婚姻保守秘密.
点评：这里for a moment可理解为暂时,即短时间的意思.
for a moment是在修饰keepingXXX a secret,这段短语.
They have their reasons for keeping their marriage a secret for the moment.
他们有自己的原因,将他们俩婚姻的那一刻保守秘密.
点评：这里for the moment可理解为某某事件的那一颗,即短时间的意思.不过,全句并不通顺,写成如下较为通顺,但意思又变了.
They have their reasons for keeping their marriage a secret down/solely for the moment.
或
They have their reasons for keeping their marriage a secret till the moment.
(他们有自己的原因,将他们俩婚姻保密到来领的那一刻.)