Connecting 5V device to 3.3V input through a single resistor
For high-impedance inputs, you can use a voltage divider to level shift downwards. The requirements are that the resistor values need to be large enough to not strain the source too much, and small enough so the impedance of the input does not affect the resulting voltage.

That usually gives you a wide range, at least if there is no internal pull-up or pull-down on that input.

The single resistor approach can work with some components, but in general it means that some transistor will have current flowing from base to collector, which is generally a bad idea, even if it is only a little. The circuit simply is not designed for this case, and what happens is therefore undefined.