Learn Danish: Jeg troede at det var advokaten der løj. - I thought that it was the lawyer who was lying.

Yes, you absolutely can! Saying Jeg troede at advokaten løj just means "I thought the lawyer lied". By using the cleft sentence structure det var... der... (it was... who...), you add emphasis. It highlights that it was the lawyer specifically who lied, rather than someone else.

Why is "der" used for "who" here instead of "hvem"? Can I use "som"?

In Danish, you cannot use hvem as a connecting word in the middle of a sentence like this. To connect the parts of a cleft sentence, you must use der or som. Because the lawyer is the subject (the one doing the lying), der is the most natural choice, though som is also grammatically correct.

Why use "troede" for "thought"? Could I use "tænkte" or "syntes"?

This is a great refresher! You use at tro (past tense: troede) for a belief or a guess that can be proven right or wrong, like guessing who lied. You use at synes for subjective opinions (like "I thought the movie was good"), and at tænke for the literal action of having thoughts in your head.

What is the infinitive form of the verb "løj"?

løj is the past tense of the irregular verb at lyve, which means "to lie" (as in, not telling the truth).