The passive voice (المبني للمجهول)

Vowels, not extra words

English builds a passive out of two words: "was written". Arabic changes the vowels inside the single verb and adds nothing at all.

Past: damma on the first letter, kasra before the last root letter. كَتَبَ → كُتِبَ.
Present: damma on the prefix, fatha before the last root letter. يَكْتُبُ → يُكْتَبُ.

Hollow verbs shorten to a kasra and a long ي: قَالَ → قِيلَ it was said, بَاعَ → بِيعَ it was sold. Their present is يُقَالُ, يُبَاعُ.

The old object becomes the subject

What was the object of the active verb is now the subject of the passive, and it goes into the nominative. Grammarians call it نَائِب الْفَاعِل, the deputy of the doer. The verb agrees with it exactly as in any verbal sentence.

• Active: كَتَبَ أَحْمَدُ الرِّسَالَةَ. — Ahmad wrote the letter.
• Passive: كُتِبَتِ الرِّسَالَةُ. — The letter was written.
• فُتِحَتِ الْأَبْوَابُ فِي السَّاعَةِ الثَّامِنَةِ. — The doors were opened at eight.
• تُسْتَخْدَمُ هَذِهِ الطَّرِيقَةُ فِي الْمَدَارِسِ. — This method is used in schools.

The doer is not mentioned

This is the rule, not a preference. الْمَبْنِيّ لِلْمَجْهُول means "built for the unknown": the passive exists precisely because the agent is unknown or irrelevant. If you want to name the agent, use the active voice. Modern journalism does write مِنْ قِبَلِ ("by"), but it is a translation habit, not classical usage, and a careful writer avoids it.

• سُرِقَتِ السَّيَّارَةُ. — The car was stolen.
• قِيلَ إِنَّ الِاجْتِمَاعَ غَدًا. — It was said that the meeting is tomorrow.

تَمَّ plus a masdar

Modern written Arabic very often replaces the passive with تَمَّ (it was completed) followed by a verbal noun. The meaning is the same and the construction is easier to read without harakat.

• تَمَّ افْتِتَاحُ الْمَكْتَبَةِ أَمْسِ. — The library was opened yesterday.
• تَمَّ إِرْسَالُ الرِّسَالَةِ. — The letter was sent.
• يَتِمُّ اسْتِخْدَامُ هَذَا الْبَرْنَامَجِ فِي الشَّرِكَةِ. — This program is used in the company.

Note the masdar takes its object into the genitive, since the two nouns form an iḍāfa.

Watch out as an English speaker

• There is no helping verb. ✗ الرِّسَالَةُ كَانَتْ كُتِبَتْ. / ✓ كُتِبَتِ الرِّسَالَةُ.
• Do not attach the agent with a preposition out of habit. ✗ كُتِبَتِ الرِّسَالَةُ مِنْ أَحْمَدَ. / ✓ كَتَبَ أَحْمَدُ الرِّسَالَةَ.
• The new subject is nominative, not accusative. ✗ كُتِبَ الرِّسَالَةَ. / ✓ كُتِبَتِ الرِّسَالَةُ.
• Do not confuse the passive verb with the passive participle: كُتِبَ is an event (it was written), مَكْتُوب is a state (written, in writing).
• Without harakat كتب is both active and passive, so a reader relies on context. When the difference matters, write the vowels.

Example dialogue

مَتَى فُتِحَ الْمَتْحَفُ؟ — When was the museum opened?
تَمَّ افْتِتَاحُهُ فِي الْعَامِ الْمَاضِي. — It was opened last year.
وَهَلْ عُرِفَ سَبَبُ الْإِغْلَاقِ؟ — And was the reason for the closure known?
قِيلَ إِنَّ الْمَبْنَى قَدِيمٌ، لَكِنْ لَمْ يُذْكَرْ شَيْءٌ رَسْمِيٌّ. — It was said that the building is old, but nothing official was mentioned.
تُسْتَخْدَمُ الْقَاعَةُ الْآنَ لِلْمَعَارِضِ. — The hall is now used for exhibitions.