Keyword Density Checker

No. Google has not used density as a signal for well over a decade. The tool is useful in one direction only: catching accidental over-repetition that reads badly to humans.

Above about 3% for a single term usually reads as stuffing. The fix is to rewrite the sentences, not to swap in synonyms mechanically - that reads worse.

Articles, prepositions and pronouns dominate any text and tell you nothing. They are filtered so the list shows meaningful terms.

Usually, but only just. If your target phrase is not in the top few at all, the page probably is not about what you think it is about.

Whatever falls out of writing normally, usually between 0.5% and 2% for the main topic. If you have to check whether you used the phrase enough, you probably used it too much.

Yes. It is an explicit spam policy violation, and beyond the penalty it reads badly, which costs conversions long before it costs rankings.

Whether the page covers the subtopics the ranking pages cover. Coverage of related concepts predicts rankings far better than repetition of one phrase.

Because real queries are phrases. Seeing that best time tracking software appears nine times is more useful than seeing software appears forty.