It is already in past tense.
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Iron must be Smelted in Oxygen free Furnace or Iron Oxide, Rust, is the Result.
A. mainly wood and sometimes iron
The End of the Iron AgeThe Iron Age began in 1200BC and ended either in 587 or 586 BC.
The past tense of "iron" is "ironed." The past participle of "iron" is also "ironed."
The past participle of "am not" is "have not been."
The past participle of "do" is "done." The past participle of "have" is "had."
The past tense of "have" is "had," and the past participle is also "had."
The past and past participle for "buy" is "bought."
Being is the present participle. The past participle is been.
The past participle is thought.
Eating is the present participle; eaten is the past participle.
"Did" is the past and "done" is the past participle.
The past participle is had.
The past participle of "do" is "done."
The past participle is done. The simple past tense is did. The present participle is doing.