en fait

Meaning

CORRECTS what was just said or what the other believes: the reality is different.

Position and punctuation

at the start of the sentence · a comma after

Example

On croit souvent que l'inscription est payante ; en fait, elle est gratuite la première année.

People often think registration costs money; in fact, it is free for the first year.

In the DELF

the B1 letter · the B2 oral presentation

The trap

EN FAIT CORRECTS, EN EFFET CONFIRMS, and that is the whole difference. English "in fact" does not split the same way.