Learn Swedish: Det viktiga experimentet utfördes av en smart kemist. - The important experiment was performed by a smart chemist.

In a passive sentence, the focus is on what happened (the experiment) rather than who did it. When you still want to include the doer (called the agent), you use the preposition av, which means "by". So, the experiment was performed av (by) a chemist.

What does the -s at the end of utfördes do?

It forms the passive voice. The regular active past tense of the verb is utförde (performed). By attaching an -s to make it utfördes, the meaning changes to "was performed".

Why are there two definite markers for "the important experiment" (Det and -et)?

This is a reminder of Swedish double definiteness. Because you have an adjective (viktiga), you must use the separate front article Det. However, the noun experiment still demands its definite suffix -et attached to the end, giving us Det viktiga experimentet.

Why does the adjective viktiga end in an -a here?

In a definite phrase like this, the adjective must take its definite form. For almost all Swedish adjectives, the definite form simply adds an -a to the end.