"The ladies wear the modern gowns."
Translation:Die Damen tragen die modernen Kleider.
Both are grammatically perfect, although it does change the meaning. Without the article, they are just wearing any modern dresses that exist in the world, of which the listener and speaker do not share any knowledge or context. With the article, there is some implied knowledge or context of the modern dresses that they are said to be wearing.
"What are they wearing?" "They are wearing modern dresses." ("the" would be wrong here, since the listener would not know which dresses are being referred to)
"What are they wearing? These retro ones or those modern ones?" "They are wearing the modern dresses." (Here we use "the" because of the shared knowledge of the modern dresses being discussed)
When the adjective is precceded by the definite article(der,das,die)or demostrative pronoun(dieser,diese,dieses) or the words welch,solch,manch, beide,alle,dies,jen. Then takes weak inflection.
When the adjective is not precceded for any article,or precceded for words of quantity as(etwas,wenig,viel,viele,einig,ein paar,ein bieschen
Then takes strong inflection
I'm a native Spanish speaker who uses English almost everyday (job, university, tv shjows, music, books...), and that's why I started studying German from English (It's almost like a mother tongue to me). And, really, I've never seen before the word "gown". It seems I'll also learn some English this way.