I was told that the amount of words one knows in English is extremely important in English conversations.
And I felt that I really understood that when I went on a trip.
ご質問ありがとうございます。
"I was told that the amount of words one knows in English is"＝「英語で知っているの単語数が～だと私は言われた」
"extremely important in English conversations."＝「英会話で非常に重要だと。」
"And I felt that I really understood that when I went on a trip."＝「そして私は旅行に行ったときに、それを本当に理解したと感じた。」
ご参考に。
アメリカ合衆国 
