Why diode voltage is equal to source voltage when source voltage is less than 0.7?
You should provide a circuit.  However, I will assume a circuit consisting of a voltage source, the diode and a resistor in series.  If the voltage source is less than 0.7 volts, then the simple model of a diode would mean that the diode is off and no current is flowing. That means that the voltage across the resistor, in accordance with Ohm's law, is zero.  In that case the voltage across the diode is equal to the voltage source.