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We propose to attend the seminar rather then go to the mall.
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The phrase is "rather than", meaning, roughly, "instead of".
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In addition,

If you see or hear a word ending in -er, use "than". For example, the word "than" is part of a set pair:

bigger than
smaller than
rather than
Students: We have free audio pronunciation exercises.
Comments  
then is equal later and after.
e.g. i went to cinema then drank cola.
but than is used for comparison.
e.g. i am stronger than you
or i am more intelligent than you...

best regards
 Casi's reply was promoted to an answer.
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