Oh hey, thanks for sending a message and thanks for using my course! That course is reaaaally old and I should look through it, in fact I had completely forgotten I’d even made it.
In correct Esperanto it can be malpurĝardeno, malpuraĝardeno, malpura ĝardeno or ĝardeno malpura. I probably didn’t put all those answers in as options. I have no idea if I wrote it in the course itself or not but the ending vowels -a -e -o aren’t just grammatical endings, they’re actually separate, unique words just as “cat” or “fear” is a unique word in English. So when we compound words, adding -a -e -o in is more like exactly specifying the relationship between the two words instead of leaving it up to common sense.
In something like malpurĝardeno common sense tell us it must be malpura, but we might get confused (not necessarily due to any irregularities in Esperanto, but due to irregularities in our brain from our own native languages) in another word, such as “miaopinie - in my opinion”. Logically there’s no reason why we can’t say “miopinie” instead of “miaopinie” but it’s just easier for most of us to figure out that it must be understood as “my opinion” and not “i opinion” if we add the -a in there.
“de vintro” and “vintra” are both equally as common (also, people can and do say malsomero “opposite-summer” for winter, or “malvintro” for summer). People normally pick which one of the two to use depending on their native language, but frankly speaking the better you get at esperanto the more you crave to make what you’re writing FASTER and SHORTER and thus you’ll more often favor forms like “vintra”. “vintres” is far less common than both those other two but i’ve been seeing it around more and more lately.
As an example of what I mean by shorter forms, I forgot if I put it in the course or not, but you can say for example:
miadomes “my house’s”, instead of “de mia domo”
la aĉetirontes hundo (aĉet-(ej)-ir-o-nt-es)
la aĉetejirontes hundo
“the (person who’s) going to go shop’s dog”, or “the dog of the (person who’ll be) going to the store”
instead of “de la ulo/homo/persono kiu iros aĉeti”, “…kiu iros fari aĉeton” or “…kiu iros al la aĉetejo”. Also, this is a case where saying “la aĉetejironta hundo”, -a instead of -es, would change meaning if context didn’t make it clear (it would then be “the DOG who is going to go to the store”, unless we already know we’re talking about a person)
I especially notice these kinds of things gaining more popularity on places like Twitter where we’re forced to reduce letter count as much as possible, but I’ve seen them in fiction and so on as well.