Learn Russian: Это старый железный ключ.

When you are stating "This is a..." or "That is a...", Russian always uses the fixed neuter pronoun Это, regardless of the noun's gender. If you used Этот, it would attach directly to the noun, making an incomplete phrase: Этот старый железный ключ... means "This old iron key..." rather than a full sentence.

In English, "iron" can be a noun. Why does Russian use the adjective железный here?

English frequently stacks nouns together to describe things (like "iron key" or "paper cup"). Russian rarely does this. Instead, you must use a relational adjective formed from the material. Here, the noun железо (iron) becomes the adjective железный (made of iron).

Is there a specific rule for the word order of the adjectives старый железный?

Yes, similar to English, Russian follows a natural adjective order. General descriptions and qualities like age (старый) or size usually come before highly specific, objective categories like material (железный) or nationality.

Why is there no soft sign (ь) at the end of ключ, since ч is a soft sound?