The key words are "on" and "in" to be "on the taxi" is understood as being physically on top or exterior of the taxi. example: "there is a sign on the taxi" to be "in the taxi" is understood as physically inside the taxi. example: "I sit inside the taxi" Hope this helps.

My spanish is not perfect... "On the taxi" - algo que está en el exterior del taxi. Something on the exterior of the taxi. "In the taxi" - algo que está en el interior del taxi. Something on the inside of the taxi. Or the state of being within the interior of a taxi. "In the taxi" is a more common phrase. "I rode in the taxi." "On the taxi" is uncommon. "I saw a red light on the taxi." The most common phrases around taxis... "I took a taxi." "I need to take a taxi." "I need to call (or get) a taxi." "I will have to take a taxi." etc.