Proof of onward travel means that airlines and governments want to make sure you are not illegally immigrating to their country, so they need to see proof of a return ticket home. Basically, some countries want to make sure you aren't attempting to move there on a tourist visa and never leave.
In most cases, yes. Depending on the fare class of the ticket, you may have to pay a fee for the change, as well as any difference in the fare. Unless you have a sale ticket that, tells you in advance that no changes are allowed, you can contact the airline company and, the most would be to pay a fee.
Return train tickets (or two-way tickets) are tickets that are valid for both an outward and returning journey from the same station or destination on your chosen route.
In cases where you do not use the departure ticket, the remaining (return) part of the ticket will be lost. The same applies to tickets with stop-overs - you must fly all portions of the ticket and cannot start the journey in the middle of the route.
Proof of onward travel means that airlines and governments want to make sure you are not illegally immigrating to their country, so they need to see proof of a return ticket home. Basically, some countries want to make sure you aren't attempting to move there on a tourist visa and never leave.
Instead of multiple itineraries and airlines, a round-trip may just make your life easier if you don't want to deal with more than one reservation at a time. In addition, two different reservations could become a hassle if one flight delay or cancellation on the outbound forces you to cancel or change the return trip.
Open Return tickets cannot be used for the same journey more than once but you may start, break and resume, or end your journey at any intermediate station.
To offer you the best price on your chosen route, we found that a round-trip ticket is actually cheaper than a one-way flight. However, the practice of “throwaway ticketing” — only using the first part of a round-trip ticket — is prohibited by a majority of airlines in their terms and conditions.
Yes you can purchase a one way ticket. There are several Low Cost Airlines in Europe such as Ryanair, Wizz, Germanwings, easyJet, Vueling, Norwegian - to name just a few. you don't say where you're starting from, but have a look at Skyscanner or Google flights and it will tell you which airlines fly that route.