Hi Thuy Nhien,
This is a very common question as 'deny', 'refuse', 'reject' and 'decline' often translate to the same word in other languages, so learners often have problems distinguishing between them.
One useful way of seeing the difference between words is to look at the opposite of each one...
'Accept' could be the opposite of 'refuse', 'reject' and 'decline', so we can see that these words have very similar meanings.
The opposite of 'deny' would be 'admit', however, so this is different to the others - an 'odd one out'.
The main meaning of 'deny' is to say that something is not true. If the police are questioning somebody, the suspect might deny that he committed a crime, for example.
'Deny' also has a less common use, which is quite similar to 'refuse' - if you deny somebody something, you 'refuse' to give it to them - for example:
"The guards denied their prisoners food and water"
Finally, if you 'deny' someone, you say that they aren't connected to you at all - but this use is rather old-fashioned.
To 'refuse' is the opposite of to 'accept' - if you refuse to do something you choose not to do it, or say firmly that you will not do it.
You could also refuse something, which means that you don't accept it. For example:
"I offered him a cold drink but he refused it"
Notice that the pronunciation has the stress on the second syllable - refuse as opposed to refuse, which is a formal word for rubbish.
'Reject' is quite similar to 'refuse' - the opposite of both would be 'accept'.
If you reject a proposal or a request, for instance, you decide not to agree with it...
"Judge Dread rejected the lawyer's request for more time to study the case"
If you reject a belief or a theory, you decide that you do not believe in it and you do not wish to follow it...
"The rebels rejected the authority of the central government."
'Reject' often carries the added meaning that you don't think something is good enough - if an employer rejects a job applicant, or a machine rejects a credit card it is because something is considered unsuitable, invalid or wrong in some way.
If someone rejects a lover, their family or friends, they behave with cruelty or indifference towards them and perhaps do not want to see them any more.
Notice that in all cases, the pronunciation is reject, reject, with the stress on the second syllable, which is common for verbs with two syllables.
The noun, a reject, has the stress on the first syllable and means somebody or something which has not been accepted. For example:
"This shirt was very cheap because it was a reject"
Finally, we come to 'decline'... 'Decline' can be a rather formal synonym for 'refuse' - if you decline something or decline to do something, you politely refuse to accept it or do it...
"The princess is believed to have declined various proposals of marriage"
for example.
It can also be a noun - but this time it is pronounced the same as the verb, decline.
Then there's the intransitive verb - that's a verb without an object.
If something declines, it loses quality, importance or strength. Listen to how the verb 'decline' is used in this sentence:
"As China and India become more powerful, the economic power of the United States may be declining"
So, in conclusion then, we might
deny an allegation
refuse an offer
reject a suggestion - and
decline a formal invitation.
Thanks for your question Thuy Nhien.