No rule for this one
No ending or word group predicts the article of Taxi. Learn it as a pair and say it out loud: das Taxi.
das Taxi
It's das Taxi. Taxi is neuter, plural die Taxis.
Ich nehme ein Taxi.
I take a taxi.
No ending or word group predicts the article of Taxi. Learn it as a pair and say it out loud: das Taxi.