1- they should send it to us once.
It should be sent to us at once.
2- the mailman is delivering the mail now.
The mail is being delivered now.
3- He has to finish it today.
It has to be finished today.
4- the police are holding him for further questioning.
He's being held for further questioning by the police.
5- They may organize a new group next week
A new group may be organized next week.
6- You ought to send the package by registered mail.
The package ought to be sent by registered mail.
7- the citizens are defending the city bravely.
The city is being bravely defended (by the citizens).
8- they cannot hold me meeting in that room.
The meeting can't be held in that room.
9- they may pay the bill before marchandise while we are our. La 2ème partie de cette phrase est incompréhensible.
The bill may be paid...
10- He has to pay the bill before the first of the month.
The bill has to be paid before the first of the month.
11- He may pay the bill for us.
The bill may be paid for us.
12+Congress is debating that question today.
The question is being debated by Congress today.
13- For the time being Karen is teaching that group.
For the time being, that group is being taught by Karen.
14- You ought to water the plant once a week.
The plant ought to be watered once a week.
15- The company is shopping the marchandise today.
The merchandise is being shipped today (by the company).
16- We must warn them of the danger.change the???
They must be warned of the danger.
17- They couldn't sell the house at that price.
The house couldn't be sold at that price.
18- They are sending my aunt to Europe on a special mission.
My aunt is being sent to Europe on a special mission.
19- You should insure the package
The package should be insure.