Question

anna was organising a birthday

Answer 1

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A birthday was being organised by Anna.

Structure ↴

Subject + was/were + being + past participle form of the main verb + Object

Explanation ↴

The given sentence is in past continuous tense and we can change it into passive voice.

At first, the object (a birthday) in the active becomes the subject in the passive voice. Next, The subject (Anna) in the active voice becomes the object in the passive voice and it is generally preceded by the preposition 'by'. After that, the main verb of the sentence is changed into its past participle form, and it is preceded by the correct form of 'be' verb in the passive voice.

Answer 2

Answer:

A birthday was organised by Anna.