"At the supermarket" and "in the supermarket" mean more or less the same. 'In' means 'inside' - in the sense of being in the interior of the building. 'At the supermarket' could mean inside the building, or it could mean that you are standing just outside the front door, or maybe in the car park.

Also: Besides being AT school, I can be IN school. I can be AT or IN a store. I would not be AT love or ON love, but I could be IN love. If I were a country, I could be AT war with another country. I could be involved IN a war with another country, but I would not be ON a war with another country. Here are some other examples and lessons: http://www.eslgold.com/grammar/in_on_at.html Also, here is a website with a little "test" so that you can see if you use the Propositions, AT, ON, and IN correctly. http://www.better-english.com/easier/inonat.htm .

Not really the same Ana. It is a bit of an abstract thing to try to teach a foreigner. The Supermarket was mentioned by Su Ki. We could say that we are AT the supermarket, or IN the Supermarket. Referring to education, we could say we are AT and IN a grade. This is a finely nuanced subject. I do not know what I can do to help. It is not easy for anyone. I recommend you study the way the words are used; but let me try by example. If I were tiny, I could be IN a bowl of Jello. I could be IN school. I could be IN trouble with teachers and administrators. I could be IN Jail or Prison. I could do something IN anger. I could be IN fear. I could be IN Space. I could be IN love. I could be IN disagreement with you. I could be IN a good mood. Are you deep IN thought? Are you deeply involved IN studying? -------------------------------------------------------------- I could be AT a stage in my life. I could be AT school. I could be AT odds with my enemies. I could be AT the Store or the Airport. I am AT the very least, generally happy if not indeed, always happy. I am sitting AT but not IN my computer. I am sitting AT, but not IN, my desk. Are you sitting AT your desk? ---------------------------------------------------- I am ON my chair. I am ON top of all of my problems. I am ON the telephone. I am ON the roof of my house. I am ON the Internet, but not AT or IN the Internet. (hope this helps. Ask for more help if you like) .

"At the supermarket" or "in the supermarket"? :)

More context needed, please!!