I am 100 % sure, you can say "in the spring" and "in spring" in the English language. Just look it up online or read some English it is defenitely used in both ways from English natives!!! "In THE spring" is even used in the oxford dictionaries! http://www.oxforddictionaries.com/de/definition/englisch/spring
I am not sure whether there is a difference in meaning or both ways are interchangeable. This might also be a regional difference in the English language. But I couldn't really find it out when I looked it up in the internet. http://www.english-test.net/forum/ftopic47002.html
Actually: If it is a regional variety, I would strongly recommend to have both ways accepted! It is very helpful, if native speakers say what they think is correct out of their gut feeling, because we non-natives have to rely on other sources (If something is written in oxford dictionaries, it should be correct, because it's an official website. If just random people comment it in an English learner's forum, I wouldn't believe everything I read there)
I would say you can use them interchangeably, but it sounds more natural to me to use "in the spring" when you mean the whole season in general, such as "Lviv is nice in the spring". If you're talking about a specific time then I would just use in, such as "we're going to Lviv in spring", because you're probably not going for the whole season, just for a time in spring. Saying "we're going to Lviv in the spring" sounds a little old fashioned or posh to me (UK English speaker).
"In the spring" is the natural way to say it in the North-Eastern United States. If you said "In spring" people would know what you meant, but it would sound slightly odd and a little pretentious. To mark "In the spring" as wrong is certainly an error that needs correcting. Yes, I've reported it.
My vote is for the use of "in the spring."
Yes, I have been reporting it. This is the third time in three translations in this lesson. You think there will me more? ;)
I am a sufficiently ;) native speaker from eastern United States.
You know, it is unfortunate that with some 25K students there are only a few of us that consistently comment on this board. However, you would think that a grammar teacher would be here and be able to settle this. :)
BUT...... maybe Konyskiw is right and it is a British thing.