Learn Italian: Avete pensato che io avessi molto entusiasmo? - Did you all think that I had much enthusiasm?

The verb pensare (to think) expresses an opinion, which triggers the subjunctive mood. Because the main verb is in a past tense (avete pensato), we must use the imperfect subjunctive (avessi) rather than the present subjunctive (abbia) to keep the timing in the past.

Why is the pronoun io included before avessi? Usually, Italian drops pronouns.

In the imperfect subjunctive, the first and second person singular forms of avere are identical (io avessi, tu avessi). Including io removes any ambiguity, making it completely clear that 'I' had the enthusiasm, not 'you'.

Is molto acting as an adjective or an adverb here?

It is an adjective here, describing the noun entusiasmo (enthusiasm). Because entusiasmo is a masculine singular noun, molto takes its masculine singular form. If the noun were feminine, like energia, it would change to molta energia.

Could we use the past perfect subjunctive here instead?