Hi Laoise,
Here is the Kwiziq lesson about the 'ne explétif' used with expressions like 'avant que' -
https://french.kwiziq.com/revision/grammar/how-to-use-avant-que-ne-expletif-subjonctif-before-i-do
and another one explaining the 'ne explétif itself -
https://french.kwiziq.com/revision/grammar/how-to-understand-the-ne-expletif
Hope this helps!