IN
Use 'in' with spaces:
Use 'in' with bodies of water:
Use 'in' with lines:
AT
Use 'at' with places:
Use 'at' with places on a page:
Use 'at' in groups of people:
ON
Use 'on' with surfaces:
Use 'on' with small islands:
Use 'on' with directions:
IMPORTANT NOTES
In / at / on the corner
We say 'in the corner of a room', but 'at the corner (or 'on the corner') of a street'
In / at / on the front