-en is used in regular, definite, masculine nouns. -et is used in regular, definite, neuter nouns. -e in this lesson is used in regular, indefinite, feminine nouns. -a is used jn regular, definite, feminine nouns.
A kind of trick to tell for masculine and neuter is if you would use "en" or "et" for the article in the indefinite form, you use it as a suffix in the definite form.
An example of each: "EN mann" would be the indefinite form of "a man" while "mannEN" is the definite form "the man".
"ET eple" would be the indefinite form of "an apple" and "eplET" is the definite form "the apple".
Feminine are a little different because in the most common forms kf bokmål, they dont use the feminine article "ei" and instead consider them masculine and use "en".
An example of this: "Jente" (girl) is a feminine noun, but most speakers/writers will not say "ei jente", but they would say "en jente".
If you come accross a masculine noun, such as this, that is actually feminine, you can typically change the "-e" to an "-a" to change from indefinite to definite.
Hope I explained that correctly. Someone correct me if I messed anything up.