This is something called "patchwriting", and it's a form of academic dishonesty.
Plagiarism is all about the originality of the work. When you take someone's original words and quote it correctly, you are being upfront about who is responsible for that sentence. When you write a completely original sentence using the same information, that's paraphrasing (and it shows you understand the information).
When you alter a sentence that you didn't write and pass it off as a paraphrase, you are implying it is original work when it is not.
This is why patchwriting is considered a kind of plagiarism even if you use your "own words". It's claiming you created something when, in truth, you only altered something that was created by someone else.
You should always think about what the sentence actually means and ask yourself if you can genuinely write a new sentence that contains the same information. If you can, do so. It's what your lecturers are hoping you will do.
If you can't write a genuinely original sentence about this information, quote the original sentence, but try to only quote the core part of the original and use your own words to introduce the quote (you should never let a quote stand on it's own, but always introduce it and/or elaborate on it).
And remember, it's the information you are citing, not the words, so always use proper referencing.
