Sophia says: Recognise the genitive absolute
If a noun or pronoun with a participle in agreement cannot be made the subject or object of the main verb, Greek uses a genitive absolute: the noun/pronoun and participle are put into the genitive case.
The easiest way to translate a genitive absolute in English is often by beginning ‘When…’
του στρατηγου κελευσαντος, απεχωρησαν.
When the general had given the order, they withdrew.