In, on, at — time and place

preposition: A preposition is a small word that places something — in space, in time, or in a relationship. It is the part of the language that translates worst: almost no preposition matches one to one across languages, which is why guessing from your own language fails so often.

the three shapes: at is a point, on is a surface, in is a closed space. The same three shapes work for time as well as for place: an hour is a point (at six), a day is a surface you stand on (on Monday), a month is a space you are inside (in May). Learn the shape once and it serves twice.

fixed expression: Some combinations follow no shape at all: at home, in bed, on foot, at night, in the end. They are not rules, they are words — learn them as whole blocks, the way you learn a word, and stop asking why.