I am sorry you are having a difficult time using the MC33926; we understand the MC33926 has a complex control interface and try to make it easier to use by listing each pins’ function and default state as well as providing basic connection suggestions on their product pages. It sounds like you might be using the MC33926 Motor Driver Carrier.
In general, there should be a common ground shared between all components in your system, so you should connect GND from your Teensy board to the MC33926.
You might find looking at the pinout table under the “Pinout” heading on the MC33926’s product page (if you have not done so already) helpful in understanding the function of each pin on the board. Like it says in the table, INV can be used to invert the motor direction if IN1 and IN2 are set to different values.
D1 and !D2, are both active by default, so you need to pull them to their inactive states to enable the motor channel. Their default (or active) states are also defined in the pinout table.
Tri-state logic, also known as a three-state logic, uses a third state, high-impedance (floating/disconnected) state, in addition to logic high (1) and low (0). When a motor driver output is at high-impedance, the motor is coasting. You can find more information about the different output operations in the truth table on page 17 of the MC33926’s datasheet.