Actually, both are correct! However, be careful with which one you use, as they mean different things! The prepositions “in” and “at” indicate whether you are referring to a location in time or in space.
“In the beginning” is used to describe the start of a period of time. It can be used alone, even if the time frame is unclear.
In the beginning, the company had all but three employees and a tiny garage as their office.
In this sentence, we can infer that “in the beginning” refers to when the company was new and had just started.
On the other hand, “at the beginning” refers to the placement of something.
You should not use pronouns at the beginning of a paragraph!
Author: Katy Lia, Cambly Tutor
If you’d like to keep this conversation going with Katy, give her a call on Cambly or make a reservation with her here: https://www.cambly.com/en/student/tutor/Katy%20Lia