After Ohm's Law, Thevenin's Theorem is probably the single most useful thing to know when

it comes to interfacing circuits together, eg sensors to amps and sensors to A/D converters.

You should study it till it's second-natured.

Then, you can extend it easily to impedances. Eg, for a ckt with capacitors, like an RC low-pass

filter, where the R is a voltage divider, for instance, you first reduce the v.div to its Thevenin

equivalent resistance, and then solve the low-pass by inspection, eg F3db = 1/(2*pi*Rth*C).