Learn Polish: Kupuję szampon w supermarkecie. - I buy shampoo at the supermarket.

Verbs ending in -ować have a special pattern in the present tense. You drop the -ować part and add -uję for I, -ujesz for you, and so on. That is why kupować becomes kupuję.

Since I am buying it, shouldn't szampon be in the accusative case and change its ending?

It is actually in the accusative case! However, szampon is a masculine inanimate noun, meaning it is a masculine word for a non-living thing. For these types of nouns, the accusative case looks exactly the same as the basic dictionary form.

Why is the ending -e added to the word supermarket?

The preposition w (in/at) requires the locative case here because it describes a fixed location where an action is happening. For most masculine nouns like supermarket, the locative ending is -e.

Why does the 't' at the end of supermarket change to ci in supermarkecie?

In Polish, adding the -e locative ending often causes the final consonant of the root word to soften. The hard 't' in supermarket softens to 'ci' before the 'e'. This is a common spelling change that makes the word easier to pronounce.