Обсуждение плана на семинаре было полезным.

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Breakdown of Обсуждение плана на семинаре было полезным.

• быть: to be
• полезный: useful
• план: the plan

Questions & Answers about Обсуждение плана на семинаре было полезным.

Why is плана in the genitive case instead of план in the nominative?

In Russian, when you turn a verb into a noun (обсуждать → обсуждение), it often keeps the same kind of object relationship, but now that object is put into the genitive case.

• обсуждать план = to discuss a plan (verb + direct object in accusative)
• обсуждение плана = discussion of the plan (noun + dependent noun in genitive)

So плана is genitive singular, meaning “of the plan.”
This pattern is very common:

• чтение книги – reading of the book
• изучение языка – study of the language
• обсуждение вопроса – discussion of the question

Using nominative план here would be ungrammatical, because обсуждение needs a dependent noun in the genitive to show what is being discussed.

What case is на семинаре, and why is it семинаре and not семинар?