"I think that you have sent the wrong letter to the wrong boyfriend."
Translation:Jag tror att du har skickat fel brev till fel pojkvän.
Both can be translated to 'that', but for different reasons.
'att' can be a CONJUNCTION similar to 'that', introducing a clause that complements a previous statement. That's happening in this case.
'som' can be used as a PRONOUN similar to 'that', indicating which thing/person is being referred to. "Det var hon som gjorde det" -> "It was she that did it."
So, they can both be translated to 'that', but only because 'that' has many different meanings!
Of course, 'att' and 'som' also have multiple meanings.
'att' can also introduce an infinitive verb, where English would use 'to'. And 'som' can be used as a comparison conjunction: "Flitig som ett bi" -> "Busy as a bee".
I'm sure there are ways to use/translate those words, as well. Silly slippery Germanic helper words.
Tycker is to have an opinion, and tror is to believe. And then just to make things more interesting, tanker is to be in the act of thinking.
Jag tycker att blå är det bäste farg -- I think blue is the best color.
Jag tror att kjolan är i garderoben -- I think the skirt is in the closet.
Jag tanker på din fråga -- I am thinking about your question.
So yes, tror is the correct verb here.
I think tycker should also be accepted.. It might be the opinion of someone (for example; your other boyfriend who thinks you should have sent hím the letter instead of the one you sent it to), it cannot be derived from the sentence whether it is more of a belief or more of an opinion. I know, not very likely, but still possible.. right?