To use 'tú' or not to use 'tú'

The perennial bug bear in Spanish of choosing 'usted' (formal) or 'tú' (informal) when addressing somebody directly rears its utterly confusing head with disturbing regularity. A minor example occurred only last night in the pub when a woman of about seventy came to bid farewell. She spoke first.

"Gracias." (So far, so good). My turn. "Gracias a usted." There was a slight pause and the hint of a mildly irritated smile. "No. A tí." The air of chastisement was palpable. It's true that the woman in question looked as if she may have been a big cheese at Woodstock in her younger days but the fact was, she was an older person and, as all the grammar books tell you, you must use 'usted' if you're dealing with an older person. They don't add in brackets, however, '(except if they're a free spirit who will be vaguely affronted by your formal language)'.

Here's the deal, as far as I understand it (which is hardly at all). When to use 'tú' (informal):

-When speaking to children, as in, "Deja tu tablet por una vez en la vida, coge tu pelota y sal a la calle para fastidiar a los vecinos como le corresponde a cualquier niño" (Put your tablet down for once in your life, get your ball and go outside to annoy the neighbours like any right-thinking child should).

-When speaking to friends, as in, "Te toca invitar." (It's your round.)

-When speaking to younger people (in their twenties/thirties) in informal circumstances as in "¿Tú sabes quién es Drake?" (Do you know who Drake is?)

-When speaking to old people who are garnished with a faint whiff of patchouli.

None of this will save you from the grey area of meeting people in their late forties/early fifties who might be equally insulted if you're too formal or, also, too informal. These situations are generally a sort of stand-off of curt utterances, each participant hoping the other will be the first to take the plunge and use the word 'you'.

- When to use 'usted' (formal): In legal situations, as in, "Pensaba que no estaba cargada, su señoría." (I didn't think it was loaded, your honour.)

- In the bank, as in, "Ustedes son unos ladrones." (You're bunch of thieves.)

- When meeting the in-laws for the first time (unless they're sparking up a spliff, obviously).

- When you want to distance yourself in conversation from someone because you don't like the cut of their jib. They may offer an opening gambit of 'tú' but you must respond with 'usted' in order to let them know you'll never be friends. However, it's important to wait a polite thirty seconds or so of further small talk before dropping the U-bomb.

Well, I hope that helps. Wishing you (both formal and informal) all a wonderful week.