Learn Swedish: Vi anser det lämpligt att utvärdera studien. - We consider it appropriate to evaluate the study.

Det acts as a "formal object" (a placeholder). The real object of the sentence is the entire action of evaluating the study (att utvärdera studien). Just like in English ("We consider it appropriate to..."), Swedish uses det to hold the object's place so the evaluating adjective (lämpligt) has something to attach to before the heavy infinitive clause arrives.

Why is the adjective spelled lämpligt with a -t at the end?

Adjectives in Swedish must agree with the noun or pronoun they describe. Here, the word "appropriate" is describing the placeholder det (which is a neuter word). Therefore, the base adjective lämplig gets a -t ending to become lämpligt.

Could I use tycker instead of anser here, since they both mean "think"?

You could, but anser is a much better fit for this sentence. Anser means "consider" or "to be of the opinion" and is used for formal, professional, or well-thought-out judgments. Tycker is used for everyday personal opinions and feelings. Since evaluating a study is a formal context, anser sounds more natural.

What happens to the sentence structure if I put the att-clause at the beginning?