Cross-country tax issues can be challanging, because they are changing and just a few people understand them completely. The most important factor in your case

Cross-country tax issues can be challanging, because they are changing and just a few people understand them completely. The most important factor in your case is "center of living", which is defined very broadly, but for ease of understanding this is the country where you spend 183 or more days/year and where your family lives and where you have most friends and most private social life. This is where you will normally pay your personal taxes... if you pay some taxes in US already, then Spain will most propably deduct them and you will need to pay only the difference between spanish taxes and US taxes (under assumption there is "double tax treaty" between US and Spain).