I find that many people use:

Please think again.

But there is already the word rethink, so why do they use think again?

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    Maybe they don't like to use 'rethink' as an intransitive verb. – We oath to creation May 4 '18 at 7:30
  • @Keepthesemind it's just a hobby? – Frank AK May 4 '18 at 7:32
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    Note that 'think' and 'rethink' may not always be interchangeable. 'I'll think about that' is English. 'I'll rethink about that' is not. (I think.) – We oath to creation May 4 '18 at 7:46
  • @Keep these mind What's the accepted stance on using 'rethink' intransitively? – Edwin Ashworth May 4 '18 at 8:50
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    @EdwinAshworth en.oxforddictionaries.com/definition/rethink says 'with object'. But merriam-webster.com/dictionary/rethink allows "He took a moment to rethink." So, I am not too sure how to deal with it. Is the object (e.g., 'the matter') perhaps left out but implied in the latter? – We oath to creation May 4 '18 at 9:56

Telling someone to think again is stronger, and it implies that you know something they don't, or they've overlooked something crucial that invalidates their conclusions. Telling them to rethink merely expresses that you're not entirely happy with their conclusion, for whatever reason.

From Merriam Webseter online I get

Definition of think again

informal —used to say that what someone believes, expects, etc., is not true or will not happen

eg. If you think you can get away with this, think again.

Definition of rethink

transitive verb : to think about again : reconsider

'Think Again" carries the certainty that the other person is wrong, whereas 'please rethink' is simply a request that they reconsider.

As an aside, I would have thought saying "please think again" is an odd construction, with the deference of the 'please' sitting rather awkwardly beside the swagger of the 'think again'

  • This may be true, but can you find supporting evidence for your claim? – Edwin Ashworth May 4 '18 at 8:48
  • @EdwinAshworth fair enough! – Bug Catcher Nakata May 4 '18 at 10:01
  • Also, rethink can be used as a noun—whereas, think again cannot be. (en.oxforddictionaries.com/definition/rethink) – Jason Bassford May 6 '18 at 18:04
  • @JasonBassford That's true, but the question is about why people would choose to use 'think again' when 'rethink' is available as an alternative---so the question only applies in those contexts were 'think again' is appropriate – Bug Catcher Nakata May 7 '18 at 1:07
  • I believe this answer is fine, but I would add that very often "rethink" takes it's reconsider notion more as a point of positive action - I feel like after you've reconsidered to decide if their is a problem, you might then rethink it to come up with a new solution. "Think again" is frequently a flippant criticism - more like "That aint going to work buddy" - if you refer "think again" to yourself, generally it is after your tried something and determined the approach clearly wouldn't work at all(more "OK that didn't work" - not really "how else") – Tom22 May 17 '18 at 23:51

As has already been pointed out in the answer from @bugcatchernakata, the two words can have different meanings, but I’d like to answer the question in a different way. In cases where the two have the same meaning, I would turn the question around:

But there is already think again, so why do they use rethink?

I do that, because ‘think again’ is the simple long-established English way of expressing the idea. Although the OED gives c.1700 as the date for the first recorded use of ‘rethink’, a Google ngram shows negligible use before the twentieth century. So I would say it is the more natural way for a native English speaker to talk, especially ordinary people.

Another point that can be made is the didactic quality of ‘think again’. The two words give more emphasis. To take an example from a (relatively modern) patriotic song, much loved by Scots (not me) at sporting occasions:

And sent him homeward,
To think again.

Now try that with ‘rethink’.

And, in general, I suspect that constructions with with ‘re-’ are later and more Latinate than a traditional everyday use of the Anglo-Saxon ‘again’. Although Humphrey Bogart never said:

“Play it again, Sam.”

he most certainly would never have thought of saying:

“Replay it, Sam.”

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