question about time descriptions
There are some words can be used to describle a period of time. but i can not distinguish them about their correct usage.
(1) a while : sometimes we say "I just fall in thinking for a while?"
(2) a moment: At that moment, we had to roly on the meager income to keep our basicly life requirements. Is this grammatically?
(3) a period of time : In that period of time, I felt very sad because one of my best friend was died.
i doubt that above three sentences are made with complete correctness of grammar. so if not, tell me why and it's better to tell me how distinguish from them three?