"The second lesson is very easy."
Translation:La deuxième leçon est très facile.
I put is seconde and it was accepted. So what’s the difference between the use of seconde and deuxième ?
The fundamental difference between “deuxième” and “second(e)” is not set in stone, but, basically:
deuxième is generally used when the list will have more than 2 (or 3) items.
second (feminine form: seconde) is generally used when it is the end of the list. (but sometimes there can be a third item too).
As we don't have enough information to know if the lessons continue (Eg une cinquième leçon) or stop at two (la seconde leçon), both are accepted (in this instance they are synonyms).
Depending on the subject matter, "une leçon" is a specific topic, while "un cours" can be a broad series of lessons or even a whole course. For example, addition of numbers would be "une leçon", but second grade mathematics would be "un cours".