Variable Trigger does not exist

The syntax is not correct. You have no Get call.

Change

Trigger.oldMap(t1.Id).IsClosed

to

Trigger.oldMap.get(t1.Id).IsClosed

It should be Trigger.oldMap.get(t1.Id).IsClosed. Because you left out the function get, Apex Code's scope resolution first tried to find a function called oldMap on the class Trigger, which doesn't exist, so it then tried to find a variable called Trigger, which also doesn't exist.