la => generally means 'to'. Indicates a destination / points to a space reference
Example : Azi am fost la scoala = I went to school today
Ma duc la mall = I'm going to the mall
in => generally means 'in'/'inside'. It's similar to english, indicates a location and the 'experience' of being there
Example : Am fost in mall = I've been in the mall (unlike 'la' which points to the fact that i'm going but .. maybe i won't go inside the mall, maybe i'll just meet someone around the mall, 'in' indicates that -going inside-)
de - generally means "by" or "of". It usually points to the type of origin
Example : Am fost surprins de rezultat = I've been surprised by the result
Am mancat un tort de ciocolata = I've eaten a chocolate cake (a cake -made of- chocolate)
din - generally means 'from'. It usually points to the place of origin.
Example : Am venit din parc = I'm back from the park
Eu sunt din Anglia = I'm from England.