beginning

× At the beginning, the Internet was only a network between a few computers.
√ In the beginning, the Internet was only a network between a few computers.

In the beginning usually means ‘initially’, ‘at first’. It is normally followed by a comma: ‘I was skeptical in the beginning, but I ended up having a Facebook account.’

At the beginning means literally ‘at the beginning (of something)’ and is usually followed by of and an expression of time or space: at the beginning of the week, at the beginning of the lesson, etc.

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