Change the following sentences from active voice to passive voice

The teacher was teaching grammar in the classroom then
I have completed the assignment
The student dissolves the substance in water
My community holds mbomuzo festival every year
The students will write the examination tomorrow

Grammar was being taught in the classroom by the teacher then.

The assignment has been completed by me.
The substance is dissolved in water by the student.
The mbomuzo festival is held every year by my community.
The examination will be written by the students tomorrow.

The grammar was being taught in the classroom by the teacher then.

The assignment has been completed by me.
The substance is dissolved in water by the student.
The mbomuzo festival is held by my community every year.
The examination will be written by the students tomorrow.