adverb
1
BAD : She has lived in New York for many years therefore she regards New York as her home.
GOOD : She has lived in New York for many years and (therefore) regards New York as her home.
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Therefore is an adverb (NOT a conjunction). It cannot be used to link clauses.
See also HOWEVER (↑ however ), NEVERTHELESS (↑ nevertheless )
2
DUBIOUS : The child looked lost and therefore I asked him if I could help.
GOOD : The child looked lost and so I asked him if I could help.
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Therefore is used mainly in formal styles: 'Crops have been badly affected by the drought and therefore food will be in short supply.'
In non-formal styles use so : 'It was quite late so I didn't stay long.'