So is there a way to distinguish between "before I sat down" and "before I sit down"? Or is it just left to the context of the sentence as to whether you're talking about the past or present?
Yes it's left to the following statements to fill the listener in on what tense it is in e.g "Cyn i fi eistedd, mi wna i fynd i nôl diod" (Before i sit down, I will go fetch a drink.), and "Cyn i fi eistedd, es i i nôl diod". (Before I sat down, I went to get a drink.)