The genitive case shows possession. In English, we show possession by using 's, s', or "of."
e.g. - the farmer's horse = the horse of the farmer
the farmers' horses = the horses of the farmers
The "of" in English is the same as 's or s'.
In Latin, we use the genitive case to show possession. There is no word in Latin that means "of." It is included in the ending. The endings are:
The problem arises...
when you have a nominative singular next to a genitive singular because they look exactly alike. Without the context of a sentence, there is no way to tell. You have to try both ways to translate and choose the one that sounds the most correct. For example:
filiae (daughter) agricolae
In theory, both words could be nominative plural (unlikely) or both could be genitive singular (equally unlikely). What you have is one genitive singular and one nominative plural. How can you tell? Try both possibilities out. Which sounds correct to you?
the farmers of the daughter or
the daughters of the farmer?
Since I know that you choose the second one, you can safely say that filiae is the nominative plural and agricolae is genitive singular. You only can tell by trying to translate these words both ways. In addition, you won't see this very often at all. Don't stress out about it now that you know how to deal with it.
NB - For now, you should translate the genitive with the word "of." Don't try to use 's or s' until I know you know what you are doing.