〜んだ・〜いだ (past): past tense (だ after ん / い)

Plain past of godan verbs ending in む・ぶ・ぬ (→ 〜んだ) and ぐ (→ 〜いだ). Here だ is the past ending, not だ "is".

飲む → 飲んだ, 遊ぶ → 遊んだ, 結ぶ → 結んだ, 死ぬ → 死んだ, 泳ぐ → 泳いだ. Same sound change as the te-form (飲んで). Other verbs take た: 書いた, 行った, 食べた. Different from the explanatory 〜んだ (= 〜のだ), which comes after a full plain form: 飲むんだ "it's that I drink", 飲んだんだ "it's that I drank".