Final answer:
The run-on sentence can be fixed by introducing a subordinating conjunction like 'although' to create a dependent clause, thus linking two related ideas in a grammatically correct sentence.
Step-by-step explanation:
To fix a run-on sentence by adding a subordinating conjunction, one must turn one of the independent clauses into a dependent clause. The corrected sentence could read: Although some of the murals painted by the Mayan artists still survive, many of the murals are in poor condition. By introducing a subordinating conjunction such as 'although', the sentence now connects the idea that some murals survive with the contrasting idea that many are in poor condition.