The Ablative Absolute is a Latin construction for which there is no parallel in contemporary English (But there is an "accusative absolute" in some dialects). As the name indicates, it has no explicit grammatical or syntactic link to the rest of the sentence, but it makes a statement that is implicitly linked to the general thought. It most commonly comes at the beginning of a sentence, setting the stage for the action that follows. The present participle, representing an action that takes place at the same time as the main verb, describes the general context within which something happens (it is very much like the ablative of time = time within which). It is often best translated by saying "While..." The past participle (or perfect passive participle), representing an action that has already taken place, describes events that precede the main event. These events are usually causal (rarely just an historical event that preceded) and so the most common translation for the past participle is "Since...." The future participle was rare in the Ablative Ablsolute until the period called Silver Latin.
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