Shouldn't this be "Jij bent een sterk jongen", not "Jij bent een sterke jongen."? I thought when the adjective is preceded by an indefinite article (een) while the noun has gender, then an 'e' needs to be added at the end of the adjective (sterk) But that isn't the case here because 'de' jongen does not have gender.
First of all, all nouns have gender - either the masculine/feminine common gender (de words), or the neuter gender (het words). I am guessing you mean neuter when you say "no gender", and in that regard you would be correct, except that jonge is not neuter (I think), and therefore should take the 'e' ending after the indefinite article.
Well, "sterk" is adjective here, describing a noun. If the definite article would be "het", then after the indefinite article, or without article, the adjective gets no e. In all the other cases it does.
"de zwarte boot" (the black boat) - "een zwarte boot"
"het zwarte schip" (the black schip) - "een zwart schip"
"de zwarte koffie" (the black coffee) - "zwarte koffie"
"het zwarte water" (the black water) - "zwart water"
As you can see, it's "het schip" and "het water", so when it's "een schip" or "water", there's no "-e" after "zwart". All the other cases it's "zwarte".
I did a "typing what she says" on this one, and she definately isn't putting stress of "jij", her emphasis is definitely on "sterke", so I thought it must be "je", since "je" is the "unstressed" version of "jij". I don't know Dutch well enough to know if this is "wrong" though, but it's certainly confusing and misleading to me as a learner.
I guess what's confusing me most, is that I was told that you say "jij" when it's emphasised or stressed, and "je" when it's not, and it's not said with very much emphasis in this sentence. Like, I interpreted this to mean that "jij" would be used in a case like "Who's a strong boy? YOU are a strong boy." But you would use "je" if it were a case like "Am I a weak boy?" "No. You are a STRONG boy." Am I correct?
The point is that language changes. A few hundred years ago, the list of pronouns would have been different. One of the last changes is that "jij", "zij", "wij" and a few related words suffer from erosion. Their vowels slowly change to schwas. That's what causes the je/jĳ pair etc.
Now, when you stress these words, you'll of course say it the right way, as it's important: "jĳ". This is what the course has taught us. There is no rule, however, that forbids saying it uneroded in other circumstances. It's quite the opposite: There are situations where you have to use the jĳ; there are no situations where you have to use the schwa. Maybe that'll be different 50 years from now, but for now -ĳ is always possible.
If you hear "je", then there can't be stress; if you hear "jĳ", then there may be stress. Here apparently, is a case where we hear and read "jĳ", but there's no stress. It sounds like the lady is trying to pronounce very carefully. (So carefully that she even says "jongèn", which is not correct.) The point is, however, that for these pronouns it can never be incorrect to use the "ĳ".