Learn Swedish: Jag tycker om den stora loppisen som du har hittat. - I like the big flea market that you have found.

In Swedish, tycker by itself usually means to think or to have an opinion. To say that you like something, you must use the phrasal verb tycker om.

Why is it den stora loppisen and not just den stora loppis?

This is an example of Swedish double definiteness. When you use a definite article like den along with an adjective, the noun itself must also take its definite ending (-en). You are literally saying the big the flea market.

Why does the adjective stor get an -a at the end here?

Because the noun phrase is definite. Whenever you use a freestanding definite article like den, det, or de, the adjective that follows it must take its definite form, which almost always ends in -a.

How does som work in this relative clause?

Here, som translates to the English word that, and it introduces the relative clause som du har hittat. It acts as the object of the verb hittat (found). Because som is the object, the subject of the clause (du) comes right after it, followed by the verb.