Translation:My friend was in New York yesterday.
why 们 - in other cases they mix singular and plural together - a man could only guess how many person is the sentence about :(
As I understand it, 了 is used for completed actions. I would say, being somewhere isn't necessarily completing something, so you wouldn't use 了.
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."
How important is the word order of the sentence? I ask because in English we could day My friend was in New York yesterday or Yesterday my friend was in New York.
DL isn't very good at abbreviations even in courses graduated from beta.