以前 means before while 以内 means within the boundary or within (less than or equal to) a set value such as time and distance.
前 means ago, in front, and forward.
内 means within, inside, less than, and among.
20分以内 is within (less than equal to) 20 minutes, but 20分内 is less than 20 minutes. 20 minutes ago is 20分前 not 20分以前. 西暦2000年以前 is before the year of 2000. 2000年前 is 2000 years ago.
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以前 means before while 以内 means within the boundary or within (less than or equal to) a set value such as time and distance.
前 means ago, in front, and forward.
内 means within, inside, less than, and among.
20分以内 is within (less than equal to) 20 minutes, but 20分内 is less than 20 minutes. 20 minutes ago is 20分前 not 20分以前. 西暦2000年以前 is before the year of 2000. 2000年前 is 2000 years ago.