く and ぐ are basically the same letter with just a little extra punctuation to distinguish “ku” from “gu”. It shouldn’t be surprising then that the casual past tenses for words ending in “ku” and “gu” are also almost the same: いた and いだ.
泳ぐ = 泳ぐ = to swim
Blue: Verbs that don’t end in “iru” or “eru” have a casual past tense based on their last syllable.
Yellow: I’ll write that down.
Blue: If the verb ends in く you replace the く with いた.
Blue: 書く(かく) becomes 書いた (かいた）.
Blue: Similarly if the verb ends in ぐ you replace the ぐ with いだ.
Blue: 泳ぐ (およぐ) becomes 泳いだ (およいだ).
Yellow: There it is!
Yellow: I was having trouble finding my pen.
Yellow: Could you start over for? I didn’t hear anything you just said.