英语belong!

英语belong!
mine
Are these your keys?Yes,they're mine.They belong to me.
his他的
Is this John's letter?Yes,it's his,It belings to him.
yours你的你们的
Are these my clothes?Yes,they're yours.They belong to you.
hers她的
Is this Jane's passport?Yes,it's hers.It belongs to her.
theirs他们的
Are these their tickets?Yes,they're theirs,They belong to them.
mine Is this your watch?Yes,it's mine.It belongs to me.
hers
Are these her flowers?Yes,they're hers.They belong to her.
your is this my boat?Yes,it's yours.It belongs to you .
his Is this Jim's phrasebook?Yes,it's his.It belongs to him.
theirs Are these hammers Frank's and Harry's?Yes,they're theirs.They belong to them.
ours Is this your car?Yes,it's ours.It belongs to us
theirs Are these the children's pens?Yes,they're theirs.They belong to them.
看上面的话了,有的句中是belong,有的句中belongs,我想问什么时候belong要+s