Learn Finnish: Portti on avattu. - The gate has been opened.

It can mean both! Avattu is the passive past participle of avata (to open). When combined with on, it describes a resultative state. It means the gate is currently open because someone opened it. In English, this translates well to "has been opened" or "is opened".

What is the difference between saying Portti on avattu and Portti on auki?

Portti on auki just states a simple fact: the gate is open. Portti on avattu draws attention to the fact that someone performed the action of opening it. You use it when the action leading to the state is important.

How do we form the word avattu from the dictionary verb avata?

Avata is a type 4 verb. To make the passive past participle (which translates to English words ending in "-ed", like "opened"), we use the passive stem. For type 4 verbs, you replace the final -a or -ä of the dictionary form with -ttu or -tty. So, avata becomes avattu.

Why is portti in its basic nominative form here?

In passive sentences, if the object being acted upon is a complete, countable thing (like one entire gate), it usually takes the basic nominative form rather than the partitive or genitive.