Hi,
imagine you're walking down the street with your friend. Suddenly, you notice a girl in a red T-shirt. You want to tell your friend something about her. Are you going to identify her as: "
The girl in a red T-shirt" or
"The girl in the red T-shirt" ? To be honest, I have the suspicion that either might be possible.
And... what if you omit the colour?
Other examples:
"
The girl with a / the mobile", "
The girl in a / the hat"...
Thanks in advance.