आप = very polite (Conjugates with हैं)
तुम = polite (Conjugates with हो)
तू = impolite (Conjugates with है)
I would describe it differently
आप formal/ polite
तुम informal (considered impolite if used with those who are not close friends)
तू intimate (used in romantic poetry) but also used to be deliberately rude (think you were just rear ended)
My understanding is that तू is also used when talking with children or Gods.
Yes you are correct. Lately तू and तुम are very much interchangeable and mean almost the same. In formal settings it must be आप just like vous in French or Usted in Spanish. In other words, one must use आप with strangers, older people, parents and teachers etc.
The previous sentence was "thum kathi ho"- which means you eat("you" here refers to a female) . Now it's "thum 'Kathey' ho". Meaning??
In English we use a He is/ We are/ You are/ They are/ I am. In Hindi these mostly match up with what we use in English वह है/ हम हैं/ आप हैं/ वे हैं/ मैं हुँ. है=is हैं=are हुँ=am. तुम is an exception because it matches up with हो which means “are” in this case.