Prepositions in English

Hello everybody !

I am pretty confused about the way the prepositions are used in English.
When should I use IN and when AT ?
Are there any rules that can help to understand ?
For example what usage is correct :
AT the moment or IN the moment ( e.g. IN the moment you finish that , you can call me)
IN the end or AT the end. ( e.g. IN the end he changed his mind and bought something else).

Thank you for help ,
Boris.

Hi Boris,

Prepositions are confusing in most languages because there are no very clear rules to help us understand which one to choose.

At the moment is the correct one and In the end is correct.Unfortunately there is no reason WHY-we just have to learn that as a phrase.For prepositions of time and place sometimes we can find a reason but for these expresssions you just need to learn them and there is no reason why.

http://grammar.ccc.commnet.edu/grammar/prepositions.htm

I hope this is helpful