When should you use smelt?

Answer 1

If you mean smelt as in the past tense and past participle of smell, then:

use the past - smelt when talking about something that happened in the past and is now finished eg The dog smelt the smoke and began barking.

use the past participle - smelt when:

using present perfect - I have smelt that smell before somewhere.

using past perfect - The lions had smelt the smoke and were now agitated.

Answer 2

You should use "smelt" when you want to convey the process of extracting metal from its ore through heating and melting. It can also refer to the act of perceiving or discerning something.