"I highlighted the changes in yellow." Is it grammatically correct?
I sent a email and I'm not sure if it's correct.
Thank you in advanced.

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Yep, it's a very common phrase in U.S. schools and offices.  By the way, why does your profile say that your English level is "native"?

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Yes. Perfect!

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I sent an email. 
Are you learning Spanish? If so, I highly recommend you to watch this video https://www.youtube.com/watch?v=v30Za-XLdTg&feature=youtu.be

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I sent an email. 
Are you learning Spanish? I highly recommend you to watch this video. It might be helpful for you.
https://www.youtube.com/watch?v=v30Za-XLdTg&feature=youtu.be

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Yes, that is correct.  But:

I sent aN email and I'm not sure if it's correct.
Thank you in ADVANCE.

집에서 편안하게 언어를 배울 수 있는 기회를 놓치지 마세요. 경험 많은 강사진을 살펴보고 지금 바로 첫 수업을 신청하세요!