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I worked hard because I really wanted to pass the examination.

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Can i change it,

replaced by therefore
1.) Therefore I really wanted to pass the examination; I worked hard.

a comma was added after therefore
2.) Therefore, I really wanted to pass the examination; I worked hard.

used "Due to"
3.) Due to really wanted to pass the examination, I worked hard.

let me know if you have a better example, please advice me

Thank you, Experts
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I worked hard, as I really wanted to pass the examination.
As I really wanted to pass the examination; I worked hard.
are
closer, IMO.

3 is wrong.

1, 2 change the meaning:
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therefore

3 : for that reason : because of that : on that ground : to that end : CONSEQUENTLY , HENCE <lost the wager; therefore they must pay> <A is greater than B, and B is greater than C, therefore A is greater than C>

(19 Oct. 2006).
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Comments  
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Thank you Expert, i would like to know it's ok if i replace "because" for "as" in your examples?

I worked hard, because I really wanted to pass the examination.
Because I really wanted to pass the examination; I worked hard.

sometimes i don't understand the usage or the difference between "As" and "Because" in this way. It looks like exactly same meaning.

actually, if you had used "because" in your examples, so i didn't have any question. i always see the word "because" uses in this way a million times.
Tommy,

If you want to combine the sentences, you might try;

As / Since / Because I really wanted to pass the test, I studied hard.

However if you want to use 'therefore' make sure that you give the reason first.

I really wanted to pass the test (reason). Therefore, I studied hard (action).

Hope that helps.
Thanks a lot
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