Learn Russian: Железо более тяжёлое, чем дерево.

When you use the word чем (than) to compare two things, the second thing (дерево) stays in the Nominative case. If you didn't use чем, you would have to put it in the Genitive case instead: Железо тяжелее дерева.

Why use более тяжёлое (more heavy) instead of just the single word тяжелее (heavier)?

Russian has two ways to make comparisons. You can use a single comparative word like тяжелее, or you can use более (more) plus the standard adjective. Both mean exactly the same thing, though using более is often a bit more formal.

Why does the adjective тяжёлое end in -ое here?

When you use the более + adjective structure, the adjective still has to match the gender and number of the noun it describes. Because железо (iron) is a neuter noun, the adjective takes the neuter ending -ое.

Wait, doesn't дерево mean "tree"?

Yes, but it also means "wood" as a material. You have to rely on context to know which one is meant. Since it is being compared to железо (iron), it makes sense that it means "wood" here.