Is it "in the grocery store" or "at the grocery store"?

Both are correct, but the context changes. Use at when referring to the store as a destination or a general location ("I am at the store now"). Use in when you want to emphasize that you are physically inside the building ("It is very cold in the store today").

Do I say "on aisle 5" or "in aisle 5"?

Native speakers use both. In aisle 5 treats the aisle like a corridor you are standing inside. On aisle 5 treats the aisle like a location on a map. "In" is slightly more common when you are physically walking through it.

What is the difference between "aisle" and "isle"?

An aisle (with an "a") is a walkway in a supermarket. An isle (without an "a") is a small island in the ocean. While they sound the same, they are homophones with very different meanings—always use aisle for shopping!

How do I use "for" when asking for help?

The preposition for shows your purpose. Use the phrase " I am looking for... " when you cannot find an item. For example: "I am looking for the baking soda; is it in this aisle?"

Should I say "next to" or "by" for item locations?

Both are used to describe proximity. Next to is more specific and means side-by-side ("The milk is next to the juice"). By is more general and means "near" ("The carts are by the entrance").

Why do we say "on the shelf" but "in the fridge"?

We use on for surfaces where an item sits on top. We use in for enclosed spaces. Since a refrigerator has a door and contains items inside, we say the milk is in the fridge, but it sits on the fridge shelf.