Does "There is the tall man outside" work? The hints for "van" in this sentence are "there is, it is, and s/he is."
No, it does not. But only because we are talking about THE tall man, "A magas férfi ". So, a few things would need to be changed for that.
The minimal change would be like this:
"Egy magas férfi kint van".
It is okay, but the word order sounds a bit strange for this purpose. (It can be perfect in another context). I would slightly rearrange it to one of these:
"Van kint egy magas férfi". - There is a tall man outside.
"Kint van egy magas férfi" - Outside there is a tall man.
"Egy magas férfi van kint". - A tall man is outside.
or some other arrangement.
No, the adjective is squeezed between the article and the noun (a magas férfi), so they belong together: "the tall man".
Adjectives - as long as they aren't used as nouns themselves - do not change with their noun.
If you use it as a noun (like egy szép - a beauty, a beautiful person), it gets the full noun treatment: szépet (acc), szépnek (for a beauty), széppel (with a beauty) and so on. Also, there are no grammatical genders in Hungarian.
és mi a helyzet a "szépek a virágok" kifejezéssel?(vagy nem így értette a kérdést?)
Ah, yes. I forgot the sentence adjectives. "[Noun] is [quality]." For those, the adjective will be pluralised if the noun's a plural. Good catch. :)