Identify the Part of Speech of the underlined word in

Answer: Adverb (adverb of time).

'Then' indicates when something happens and in this sentence it modifies the verb phrase 'shall be done'.

• How to test: if a word answers the question 'when?', it is usually an adverb of time.

• Example in the sentence: 'It shall be done by then.' Here 'then' answers 'when will it be done?'

• Contrast with other parts of speech: 'then' does not name something (noun), replace a noun (pronoun), or describe a noun (adjective).

Therefore, 'then' is an adverb of time because it tells when the action will occur.