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Please have a look on these sentences:

1: Making a promise also entails the risk of not being able to fulfill it.
2: Making a promise also entails the risk of being not able to fulfill it.
3: Promise me that you will never go to play cricket in the evening again. Okay, I promise (followed by muttering) but only not to fulfill it (later).

Question 1:
I think the both versions, '1' and '2', are correct. Is there any subtle difference?

Question 2:
Is the muttered part in '3' correct? The promiser is sure and determined not to be bound by any false promise he has been forced to make under the circumstances/obligation. Do I make any sense.

Please help me with the above questions.
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1 and 2 both make sense and mean the same thing. However, I find 2 to be awkward.

3 doesn't make sense to me. If he doesn't intend to keep his promise, he should cross his fingers instead of muttering. Emotion: hmm
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Hi,

As Philip points out, it is more common to say 'keep a promise' than 'fulfill a promise'.

Clive Emotion: smile
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Comments  
1: Making a promise also entails the risk of not being able to fulfill it. OK.

2: Making a promise also entails the risk of being not able to fulfill it. No.
2b: Making a promise also entails the risk of being unable to fulfill it. Better than 2.
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3: Promise me that you will never go to play cricket in the evening again. Okay, I promise (followed by muttering) but only not to fulfill it (later).

More realistic:

-- OK. I promise. [muttering to himself any number of things you might invent]

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But maybe your intent was just to check the grammar of but only not to fulfill it later. It's not ungrammatical, as far as I can see, but it is awkward to use but only not followed by an infinitive. Usually, to use only that way, you would say something like this,

Jerry made a promise, only to break it later.

CJ
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Thanks, everyone.

Hi CJ
CalifJim2: Making a promise also entails the risk of being not able to fulfill it. No.
It's not that I don't believe you but I would like to know what rule of syntax made you to rule out the second as an incorrect variant?
CalifJimMore realistic:
-- OK. I promise. [muttering to himself any number of things you might invent]
Except the one I have already proposed!
CalifJimBut maybe your intent was just to check the grammar of but only not to fulfill it later.
The above sentence doesn't sound natural to me, of course only to me. Mind you I'm just a learner. I would have written it somewhat like:
But maybe your intent was just the grammar of but only not to fulfill it later to be checked.

Your version sounds to me as if I was the one who was to check the grammar.

I hope you can see the underlying hindrance to my understanding of the structure.
CalifJimit is awkward to use but only not followed by an infinitive.
I'm not saying Google is a reliable source for correct grammar, I get more than three million hits for 'but only not to':
http://www.google.com.pk/search?hl=&q=%22but+only+not+to%22&sourceid=navclient-ff&rlz=1B3GGLL_enP...

Best wishes
Jack