Disclaimer This advice is based on personal opinion only and is designed to help you. However, Clifford Evans is not responsible or liable for any actions you do or do not take as a result of reading this article. Always remember to take an independent expert with you when purchasing a piano.
Escapement on digital pianos This is only obligatory for advanced students. Escapement is the part of the piano action allowing the hammer to return to a temporary resting place after you have depressed it. This means that you can play the bed of the key - meaning that you re-depress the key before it has cone completely up to the surface and use this escapement mechanism to play repeated notes more quickly and above all quietly. Very useful, for example in part of Fur Elise by Beethoven. Now that electronic pianos have escapement, they are that much closer to acoustic pianos - a significant improvement.
Sustaining pedal The Right pedal is essential on your keyboard. Make sure that it's one which is like a lever (down is ON and up is OFF) and not like an ON/OFF switch. The right pedal - sustaining pedal - of your electronic keyboard or piano is the one which makes the notes carry on playing even if you take your finger off them. The notes will continue to sound until you lift your foot off the sustaining pedal
Una corda or soft pedal Left pedal - useful but not obligatory. If you have one, it should be working properly by making the notes quieter. The una corda pedal is so called because on acoustic grand pianos it moves the whole keyboard so that the hammers play only one or two strings instead of two or three. On upright acoustic pianos there is a different mechanism which does not make the keyboard move. Not all electronic instruments have a una corda, and if they do - the keyboard doesn't move. You can manage very well without a una corda pedal.
Sostenuto pedal Middle pedal - useful but not obligatory. This is depressed after you have played the note, but whilst it is still held down, thereby enabling you to make certain notes last longer whilst you play other notes affected by the right or sustaining pedal.
Audio outputs Useful but not obligatory. These are so that you can connect to other things like amplifiers. Midi IN, OUT and THROUGH usually 5 pin din type sockets or USB so that you can connect to other equipment like computers, midi keyboards, sound modules and sequencers. The midi connection system is invaluable if you want to compose music, or to link several keyboards or instruments.
Earphones Useful for practising if your family is watching television. Good practice to use them in Skype lessons to avoid risk of feedback. Earphones are also helpful if you don't want anyone to hear you practise the new piece until it is ready, or the tricky passage which hasn't quite clicked into place yet. Use earphones carefully, however, because prolonged use is not good for the human ear, which has evolved a lot more slowly than modern technology. Many manufacturers are now putting warnings on their products. Professional sound-engineers only use earphones very sparingly, probably to look after their hearing and only to pick up very tiny details - the audio equivalent of a magnifying glass if you like. So prolonged use of earphones is not a good idea.