Breakdown of Чертёж, обсуждаемый архитекторами, очень сложный.
очень
very
сложный
difficult, complex
архитектор
architect
обсуждать
to discuss
Questions & Answers about Чертёж, обсуждаемый архитекторами, очень сложный.
How would we say this using a который clause instead of the participle?
You would use an active construction: Чертёж, который обсуждают архитекторы, очень сложный. Notice how the passive participle обсуждаемый turns into the active verb обсуждают, and the instrumental agent архитекторами becomes the nominative subject архитекторы inside the clause.
Why is the present passive participle обсуждаемый used here rather than a past tense form?
Because the action is ongoing at the same time as the main sentence (it is being discussed right now). Present passive participles are formed from imperfective verbs like обсуждать. If the discussion were already finished, you would use a past passive participle from a perfective verb, like обсуждённый (the discussed blueprint).
Why is the word архитекторами in the instrumental case here?
In passive constructions, including passive participle phrases, the agent (the person or group doing the action) is always put in the instrumental case. Since the architects are the ones doing the discussing, архитектор becomes архитекторами.
Do we always need commas around participle phrases like обсуждаемый архитекторами?
You need them here because the participle phrase comes directly after the noun it describes (Чертёж). If you moved the phrase before the noun to say Обсуждаемый архитекторами чертёж очень сложный, you would drop the commas.