It doesn't necessarily mean that I wasn't able to completely follow the conversation.
ご質問ありがとうございます。
「話の流れが全くわからなかった事はない」は英語で「It doesn't necessarily mean that I wasn't able to completely follow the conversation.」と言えます。
「~ない事はない」は英語で「It doesn't necessarily mean that...」を表します。
先生との英会話の例:
Teacher: During our previous lessons, what percentage do you think you were able to understand? (昔はレッスンの何割位理解できてたと思う?)
Student: Possibly 50%. But it doesn't necessarily mean that I wasn't able to completely follow the conversation. (50%くらいかな、でも全く話の流れがわからなかった事はないよ)
ご参考になれば幸いです。
I've never completely lost track of the conversation.
"I've never completely lost track of the conversation."
「会話の流れを完全に見失ったことは一度もない」という意味になります。
・I've never は「一度もない」という否定の意味を強調します。
・completely lost track of は「完全に見失う」という意味です。
・the conversation は「会話、話の流れ」を指します。
"I think I understood about 50% of the lessons back then, but I've never completely lost track of the conversation."