Is previously correct grammar?

"Previously" is an adverb, not a complete sentence. It describes something that happened before something else.

To use "previously" in a grammatically correct sentence, you need to include a verb and a subject.

For example:

* Previously, I went to the store.

* She had previously completed her homework.

Let me know if you have a specific sentence in mind, and I can help you determine if it is grammatically correct.