that vs this

The core difference is distance

Both words point at something, but they point from different ranges. This is the near word: the thing in your hand, the thing happening now, the thing you are about to say. That is the far word: across the room, last week, the thing someone else just said.

The distance need not be physical. Time and conversation work the same way. This morning is the one you are living through; that morning is one you are recalling.

Using each one

• Near in space: This chair wobbles. / Could you pass me that chair by the window?
• Near in time: This week has been brutal. / That summer, nobody had air conditioning.
• Coming up vs already said: Listen, this is important: the deadline moved. / The deadline moved — that changes everything.

That last pair is the one writers get wrong most often. This introduces what follows. That refers back to what has just been established. If your sentence is reacting to the previous sentence, that is usually the word you want.

The test

Say the sentence out loud and see whether you could add here or there. If here fits — 'this box here' — use this. If there fits — 'that box there' — use that. For ideas rather than objects, ask whether you are pointing forwards (this) or backwards (that).

The edge cases

Speakers often use this to introduce something new into a story even when it is neither near nor present: 'So this bloke walks in with a ladder.' That is normal spoken English, not an error, though it reads as casual in writing.

Telephone conventions differ and confuse people. In British English the caller asks 'Is that Ruth?' and Ruth answers 'Yes, this is Ruth.' The person speaking is this; the person at the other end is that.

Finally, watch out for a vague this or that with no clear referent: 'Sales fell and two managers left. This worried the board.' Which part worried them? Add a noun — 'this pattern', 'that departure' — and the sentence stops wobbling.