Complex sentence: They come across a lava field and attempt to drive across it but they get stuck halfway Harry trie to backup but the tires were melted to their horror fresh lava was starting to flow down the hill.
Split into simpler sentences: They come across a lava field and attempt to drive across it but they get stuck halfway. Harry tries to back up but the tires are melting and, to their horror, fresh lava is starting to flow down the hill.