"We" is commonly used in scholarly writing even if it is a single-author paper, as "we" refers to "the author and the reader." Thus, generally speaking, it is not grammatically incorrect to use "we" in a single-author paper, provided the context is right. For instance, it is fine to say "using this theory as a premise, we can definitely conclude that..." The usage of the authorial "we" is widely accepted and preferable in scholarly writing, as it is a more inclusive form of expression.
However, you have to be really careful of the context when using "we" in a single-author paper. In some cases, using "we" might sound awkward; for example, when you are thanking someone for their contribution to your paper.
When referring to an experiment that you have conducted or data that you have collected, you should use "I," not "we." For example: "I collected samples of x from 10 different patients using...." Using "we" in this case might give the impression that someone helped you with the experiment. Another option in such cases, where you are unsure, is to use the passive construction: "Samples of x were collected from 10 different patients."
It is also preferable to use "I" when expressing your personal opinion as opposed to the general view about something: "While the commonly accepted view about this is..., based on my findings, I can argue that...."
The key is to achieve a good balance between the two. Always keep the context in mind while choosing your pronoun. Use "we" for general statements as far as possible, switching to "I" when the reference is specifically and exclusively to the author.
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