Learn Hardcore Italian: Non si vede la porta invisibile.

It’s the impersonal/passive si (also called si passivante). It makes the verb impersonal or passive, so the sentence means “the invisible door cannot be seen” or “you can’t see the invisible door”, without specifying who is doing the seeing.

Is si here a reflexive pronoun?

No. A reflexive si indicates that the subject performs an action on itself (e.g. si lava “he/she washes himself/herself”). In this sentence, si doesn’t refer back to the subject; it marks an impersonal or passive construction.

Why is la porta placed after the verb instead of before?

In impersonal/passive si constructions, the logical subject (la porta) often follows the verb. Italian commonly uses the pattern Non si vede + subject to express “the subject is not seen.”

Could we also say La porta invisibile non si vede?

Yes. Both orders are grammatically correct. Putting La porta invisibile at the beginning simply shifts the emphasis onto the door itself, but the meaning remains “the invisible door can’t be seen.”

Why is invisibile placed after porta?

Most Italian adjectives follow the noun they modify. Post-nominal placement is the neutral, standard order. Only a few adjectives regularly precede the noun for stylistic or emphatic reasons.

Can we drop the article and say Non si vede porta invisibile?

No. In Italian, singular countable nouns generally require a definite article. You need la before porta to indicate “the door.”

Is there another way to express the same idea without using si?

Yes. You could use a true passive voice:
La porta invisibile non viene vista.
Or you could specify the subject:
Tu non vedi la porta invisibile.
Each version slightly changes the focus or clarity of who’s doing the seeing.

What tense and person is the verb in Non si vede?

It’s the third person singular present indicative of vedere. In impersonal/passive si constructions, the verb agrees in number with the logical subject (here la porta), so it stays singular.