Learn Swedish: Jag tycker att marinaden i grytan är stark. - I think that the marinade in the stew is spicy.

In Swedish, tycker is strictly used for expressing a personal opinion, meaning you have tasted the stew and are stating your subjective judgment of it. If you hadn't tasted it and were just guessing it was spicy, you would use tror (to believe/guess). Tänker is used only for the cognitive act of thinking.

Stark usually means "strong". Does it always mean "spicy" when talking about food?

Yes, when referring to food, stark is the standard word for "spicy" or "hot" (like chili). If you want to describe a flavor as highly concentrated or rich but not spicy, you might use a word like kraftig (robust) or simply say that it tastes a lot (smakar mycket).

How could I apply this lesson's grammar to say "a kind of marinade" instead of "the marinade"?

You would replace marinaden with either en sorts marinad or ett slags marinad. A key B2 detail to remember is that even though marinad is an en-word, it is perfectly correct to say ett slags marinad. The article ett is locked in agreement with the historical noun slags, ignoring the gender of the actual noun that follows it.

Can the word att be dropped in this sentence, just like "that" in English?