訂正<ていせい>correction
ありがとうございます→よろしくお願<ねが>いします。=I hope you will take care of me.=Thanking you in advance. To sayありがとうございますis strange because you haven't known whether someone gives you a favor. It might be a correct expression in English or Chinese. But in Japanese, we do not say "ありがとうございますThank you" before we know someone gives us a favor.
虽然你还不知道有没有亲切的人,你现在写「谢谢」。这是奇怪。我们(日本人)不做这样的东西。
回答<かいとう>answer
すみません[sɯmimaseɴ](formal) → すいません[sɯimaseɴ](casual)
You wrote " Is it because that when you say 'すみません' very quickly, then it may become 'すいません'?". I do not agree with you. I guess the reason that it is because the word is very frequently used. I think that it was reducing the burden of pronunciation by dropping one consonant. We can also find the same phenomenon in other expressions. (ありがとう)ござります<Thank you>[gozaɾimasɯ]→(ありがとう)ございます[gozaimas]・いらっしゃりませ<Welcome to>[iɾaɕɕaɾimase]→いらっしゃい(ませ)[iɾaɕɕai(mase)]・くださりませ<Give me, Please>[kɯdasaɾimase]→ください(ませ)[kɯdasai(mase)]