Sign up and Register are for new customers/users when they come to a website. Once you have registered or signed up, you then only need to sign in or log in. For example, you have registered/signed up to Italki. Therefore, you sign-in or log in every time you use it.

idk i want to learn other languages but i only know english and it put it as learning english

"log in" and "sign in" are both the same if you are referring to apps or webpages (computer-related services). They just mean to type in your user name and password and "enter" the website or service. "sign up" and "register" also both mean the same thing. They mean, to give the required personal information (usually name, the username and password you would like, maybe credit card number if it is a paid service) in order to "join" the service or website so that you can use it again in the future.

Sign up for classes already paid for

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