"I wasn't able to follow the English but I understood from seeing the video."
- "I wasn't able to follow the English but I understood from seeing the video."
"I wasn't able to follow ~" 「〜は聞き取れなかった」
"the English" 「英語は」今回の場合には "what they were saying" を代わりに使えます。
"I wasn't able to follow what they were saying but," 「何を言っていたのは聞き取れなかったが」
"but ~" 「が〜」
"I understood ~" 「〜わかった」
"from seeing the video" 「映像を見ていたら」