Question

iron is a useful metal.
adj in it? ​

Answer 1

Answer:

No. You use the definite article hen you are talking about something specific. For example, you might say, “The iron used in that building is very trong.” If you are talking about iron in general as in your sentence, you do not use the definite article. You should say, “Iron is a useful metal.”

Answer 2

useful

is an adjective...