Should I buy a one way or roundtrip plane ticket?

Countries generally require evidence of your plan to leave, such as a return or onward ticket. Make sure that you have a visa that will allow you to stay that long, and then check the entry requirements. If you will not be required to show a return or onward ticket, then a one-way ticket is fine. Most airlines don't publish schedules and fares more than 9-11 months in advance (varies by airline), so with your current plan you will not be able to purchase a round-trip ticket.

You have no choice but buy now a one-way ticket for your trip to Spain in August 2018. You are actually very late. So, don't wait further.

Air schedules for July 2019 will be released too late for you. At the beginning, some flights may also be over-prized due to a lack of competition..

Buying two one-way air tickets may be expensive. However, if, by chance, you may travel with a discount airline, two one-way tickets or a roundtrip would have rather similar.prices (travel with heavy luggage may be however expensive at a discount airline).

Get the cheapest air tickets:

http://www.willgoto.com/1/023798/liens.aspx

More about one-way air tickets:

http://www.willgoto.com/1/147220/liens.aspx

.Edit

I presume you are going to travel for a one-year shool year or a one-year iob so that you are OK as for entry requirements without a return air ticket to your country.

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Do you have a visa that will let you stay that long?

In years past, buying a round trip saved money but that's not the case anymore.

And with enormous fees to change tickets, it can be a lot cheaper to keep each way separate.

If you're not a frequent flier loyal to one airline, it's often cheaper to buy tickets on multiple airlines.

Just be very careful and read all the terms and conditions before you purchase.

Obviously, you'll need to make sure you have it prior to arrival as you may be required to provide evidence of your return ticket to clear customs

Your concern is quite valid. You should buy a one way ticket now and don't worry about the July 2019 ticket until next year.