Long-tail keywords usage

So I just started writing content for my blog using WP, I am using Math SEO and targeting some long tail keywords. The issue is I'm not sure i'm using them correctly because I usually rephrase some of them to fit them into the content or the title, does it matter? Because Math SEO doesn't seem to like it when I change the keyword phrasing/structure, usually it's a preposition I add to make the sentence grammatically correct.