"My friend was in New York yesterday."
Why is 了 not used with 在?
I think of it this way: I don't say, "I was atted New York yesterday." In Chinese "to be" verbs remain the same regardless of the time frame. 在 is a verb that includes "to be"in its meaning along with "at." So I literally say with 在, "Today I am at; tomorrow I am at; and yesterday I am at."