That's why means for that reason, so, which is why. First you name the cause, and That's why introduces what came of it: I missed the bus. That's why I'm late. If a cause follows, you need That's because. People say it in conversation and write it in emails; in an IELTS essay the contraction sounds informal, so This is why or For this reason is better.
How would you use that's why?
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Cause. That's why + result; …, and that's why + result; Result. That's because + cause; This is why + result (formal)
- The printer was broken all morning. That's why I didn't send you the contract.
- Rents in the city centre are very high. That's why many young people live in the suburbs.
- I grew up by the sea, and that's why I still prefer beach holidays.
What changed
First versionOur flight was cancelled. That's because we spent the night at the airport.
RevisionOur flight was cancelled. That's why we spent the night at the airport.
That's because introduces a cause, so the sentence said the flight was cancelled because we spent the night at the airport. It was the other way round: the night at the airport was the result of the cancellation. A result is introduced by That's why, so because was replaced with why.
That's because means “it's because”: first the result, then the cause. I'm exhausted. That's because I worked until midnight. With That's why, the same events go in the opposite order: I worked until midnight. That's why I'm exhausted. A simple check: after why comes what happened, after because comes what caused it.
Your turn
The office was closed for repairs. ___ we all worked from home that week.
Show the answer
That's why — The closed office is the cause, and working from home is what came of it. A result is introduced by That's why. That's because would mean the office was closed because everyone worked from home.
Write about a decision you made at work or in your studies. First name the reason, then give the decision itself with That's why.
My English classes always clashed with late meetings at work. That's why I switched to an online course I can take at weekends.
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