Why diode can't use as amplifier?

A2. For amplification to take place, one needs three terminals; an input, an output, and a control signal.

This indicates the original " valve" naming; after a mechanical valve, where a small effort on a (hydraulic) valve may be used to control a larger effort of the controlled fluid.

A1. If u meant why diode can't be used as an amplifier diode is used as a rectifier..

it converts ac current to dc and there is an inbuilt voltage drop... so it can't be used as an amplifier