In Swedish, almost every adjective changes its ending based on the gender and number of the noun it describes.
There are a few exceptions, but these are the rules that most adjectives follow:
For "En" nouns, no change is made; "Lång"
For "Ett" nouns, you add a T; "Långt"
For plural nouns, you add an A; "Långa"
There's also the definite form, which in almost every case is identical to the plural form.
It's used when the adjective is right before a definite noun.
"Det långa brevet" -> "The long letter"
When used as an attribute that is correct, it would be "Det långa brevet / De långa breven / Den långa mannen" (ett/plur/en). Here it is not an attribute, so "det" is not needed, and it becomes "Brevet är långt / Breven är långa / Mannen är lång". Stating the obvious, you could say "Det långa brevet är långt."
You can say "Det brevet är långt" but then it means "That letter is long".
I cannot hear any difference between lang and langt, is the t silent and there for formal grammatical reasons rather than having a phonetic difference. 'Lang' is found in many northern placenames (Langdale) and still sounded in dialect and of course still said in broad Lowlands' Scots, but as grammar simplified, no 't' sound. Maybe once a lang time ago...