AS mainly shows the present tense of the action, while ANT and AT show the duration or repetition of the action. IS shows that the action occured before the present, while INT and IT show that the actions was completed possibly before another action...
This is what I got about usages of participles on lernu.net. I got to know that the participles and indicative verbs have differences. However, when we implement passive voices, the differences seem to be blurred.
Ŝi skribas la leteron.
"writing" happens at present, or as a habitual action.
La letero estas skribata.
According to the definition on lernu.net, "writing" as a participle, indicates duration or repetition.
So what if I just want to simply express a present action in the passive sentence, not to emphasize that it’s happening now or it’s repetitive?