I didn’t use the return portion of my non-refundable airline ticket. 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, it can often be applied towards a new ticket under certain conditions. Again, these vary, but among the most common are that it must be on the exact same routing and within 30 days of the original date of travel.