How are airlines obligated to inform you of a cancellation?
 

I booked a flight with Air India, the first of 4 flights, which was cancelled. Air India never informed me. It turned out the website I had booked through - edreams - had included a notice at the end of a confirmation email, advising me to contact them as a flight had been cancelled, but I never saw it. Would anybody know if an AIRLINE is obliged to tell you, and how - phone / text etc - if they cancel a flight? I therefore missed all of the flights, bought new flights costing €800, and I have received nothing from Air India. Thank you.

If the airline cancels your flight, you should be able to easily obtain a refund. Try contacting Air India. If you bought your ticket through a "bucket shop" (deeply discounted fare) they will probably refer you back to the issuing agent, but you should still get some refund.

You bought the ticket from a 3rd party and they paid the airline. The first line of defense is that the airline notifies the purchaser of the ticket (in your case the 3rd party) and they inform YOU.. Which it looks like they did but you didn't read the e-mail. Did the airline actually have your contact details?

Yes I agree with this. I would be very surprised if this wasn't the case.