I didn’t use the return portion of my airline tickets. Can I apply it to another ticket?

Rules vary airline to airline, and more importantly, ticket to ticket. Seems like if you used only half the ticket you would have half of the value left, but unfortunately it doesn’t work that way. Instead the airlines calculate what the cost would have been to buy just the one-way ticket you actually flew, and in many cases the cost is equal or more than the cost of the roundtrip ticket you purchased so there is no remaining value. In cases where there is a remaining value after that calculation is made, the value can often be applied only to the exact same routing within 30 days of the original date of travel.