Time Expression with через and назад.

1. The preposition через is used to express time from now. The period of time is in the accusative case.
   

   через год	in a year (a year from now)
   через неделю	in a week
   через час	in an hour

2. If через is used with a time expression that includes a number, then the endings will be determined by the number. Review tutorial on Numbers and case.
   

   через два года	in two years, two years from now
   через пять часов	in five hours, five hours from now
   через три недели	in three weeks, three weeks from now

3. The preposition через always precedes the time expression
4. The preposition назад is used to express time before now.
   

   год назад	year ago
   недель назад	week ago
   час назад	hour ago

5. If назад is used with a time expression that includes a number, the endings will be determined by the number.
   

   2 года назад	2 years ago
   3 недели назад	3 weeks ago
   5 часов назад	5 hours ago

6. The preposition назад always follows the time expression