.

  1)big/traffic congestion/can/what/do/to/we/reduce/cities/in?

=> what can we do to reduce traffic congestion in big cities?
2)think/I/it's/time/to/do/to/reduce/in/something/traffic jams/Viet Nam.

=> I think it's time to do something to reduce traffic jams in Viet Nam.
3)used/school/on/Mai/foot/she/to/go/to/was/in/when/primary school. 

=> Mai used to go to school on foot when she was in primary school 
4) there/city center/traffic jams/are/the/in/rush hour/often/the/in. 

=> there are often traffic jams in the rush hour in the city centrer
5)about/people/use/in/worry/the past/traffic/didn't/congestion/to.  

=> people in the past didn't use to worry about traffic congestion
6)wear/a/helmet/compulsory/is/when/in/it/to/riding/Viet Nam/a/motobike.           

=> it's compulsory to wear a helmet when riding a motorbike in Viet Nam