Use of the instrumental case with adjectives in the past tense

Question

Is it correct and usual to use the instrumental case with adjectives when speaking in the past tense? Например:

Я был на конференции, которая была очень интенсивной.

Why not интенсивная?

Answer 1

Formally, быть (to be) requires the instrumental when used in the past tense:

• Работа тяжелая ("the job is hard"). The linking "to be" is omitted in the present tense, the adjective is in the nominative).
• Работа была тяжелой ("the job was hard"). The linking "to be" is in the past tense, the adjective is in the instrumental).

However, colloquially, many people are fine with nominative in the past as well:

• Работа у меня была тяжелая (my job was hard)

Back to your example, "интенсивный" doesn't make much sense when applied to a "conference", but it's perfectly fine to say/write:

• "конференция была интересная" in a colloquial context, i.e. talking to a colleague
• "конференция была интересной" in a formal context, i.e. writing a report.

Answer 2

While this construction is grammatically correct, it sounds awkward and unusual to me because of unnecessary complication. In 95% of cases Russian would rather say:

Я был на очень интенсивной конференции.

If you want to describe where have you been (at the conference). If you want to emphasize that this conference was very intensive, you'd say:

Конференция, на которой я был, была очень интенсивной.

Конференция, на которой я был, была очень интенсивна.

Another example:

Я съел пиццу, которая была очень вкусной.

is unusual. You'd rather say simply:

Я съел очень вкусную пиццу.

Пицца, которую я съел, была очень вкусной.

Пицца, которую я съел, была очень вкусна.