No rule for this one
No ending or word group predicts the article of T-Shirt. Learn it as a pair and say it out loud: das T-Shirt.
das T-Shirt
It's das T-Shirt. T-Shirt is neuter, plural die T-Shirts.
Ich brauche ein T-Shirt.
I need a T-shirt.
Das T-Shirt ist blau.
The T-shirt is blue.
No ending or word group predicts the article of T-Shirt. Learn it as a pair and say it out loud: das T-Shirt.