You would use "opened" as a past participle, as in "He had opened the door."
In English still and remains mean exactly the same thing in this sentence context
Can this sentence be used in a figurative/ metaphorical sense i.e. the door remains open if you change your mind?
"aprire" is the infinitive form of the verb => "to open" , in English.
"apre" is the 3rd person singular conjugation of the present tense indicative form of "aprire" => he opens, she opens, it opens.
io apro, tu apri, lui/lei apre, noi apriamo, voi aprite, loro aprono
"aperto/aperta/aperti/aperte" are adjectives and have to match what they are describing in gender and number.
Il porto è aperto. The door is open.
I porti sono aperti. The doors are open.
La scatola è aperta. The box is open.
Le scatole sono aperte. The boxes are open.