"I completely underestimated that test and couldn't do it at all."
- "I completely underestimated that test and couldn't do it at all."
"completely" 「完全に」
"underestimated ~" 「〜をたかを括った」
"that test" 「今回のテスト」
"couldn't do it at all" 「全然で出来なかった」
"at all" 「全然」