Vo in this simple circuit with forward biased diode
First, if your diode is on (you haven't analyzed the circuit yet, so you don't know, but you can assume and then check), you have a total voltage in your circuit of 3.3V (=4V-0.7V). Then, you can treat the circuit like this:







simulate this circuit &ndash; Schematic created using CircuitLab

Now, you can find the current in your circuit. Your total voltage is 3.3V (remember, the diode 'voltage source' is in the opposite direction as the main supply, so they subtract), and your total resistance is 4kΩ. This gives you 3.3V/4kΩ = 0.825mA. Since this is positive, you can say your assumption before was correct and the diode is on. If you want to be more precise, though, this amount of current is low enough that the diode voltage will probably vary a fair bit from 700mV--however, unless you're trying to make things hard for yourself, 700mV is probably a good enough assumption.

Now you have the current, so Vo should be easy to find with Ohm's law: V = IR. Vo, then, is 825μA·1kΩ, or 825mV.