"거북이의 다리가 긴데 느려요."

Translation: The turtle's legs are long, but it is slow.

[wraith89]

"The turtle's legs are long but slow." <-- this was marked wrong.

[LastSeong]

the turtles legs are long but it (the turtle not the legs) is slow

[kt-ruckus]

But how do you know which noun the final verb is referring to?

[Thiago26721]

In that case, these are not verbs, they are adjectives. They refer to the noun that has the 이/가 markers. Here, the "turtle's legs".

[Staffan431948]

It didn't accept "it's" instead of "it is." Seriously...

[oee16]

The confusion here is due to the English translation.

-ㄴ/는데 is a subordinate conjunction of concession linking two clauses, one being of contrary reasoning to the other.

• -ㄴ/는데 = but still (despite being), subordinate conjunction of concession

거북이의 다리가 긴데 느려요 = The turtle's legs are long, but still it is slow (= Despite having long legs, the turtle is slow)

• -지만 = but (except for) is a coordinate conjunction of contrast.

거북이는 다리가 길지만 느려요 = The turtle has long legs, but it is slow (= The turtle has long legs, except that it is slow )