"The Dutch buildings are tall and pretty."
Translation:A holland épületek magasak és szépek.
Because it's right before the noun (an attributive adjective).
Only predicative adjectives, or attributive adjectives without a noun (szépek "pretty ones"), get pluralised.
Because van / vannak is omitted when you use an adjective to describe the quality of something (tall, pretty, blue, important, ...).
It's necessary when you talk about where something is, but not for this kind of sentence.
Could you please explain why it is not possible to say ' magas és szép' ?
Because these are predicate adjectives. The subject is plural, so these must be plural too.
Az épület magas és szép. = The building is tall and pretty.
Az épületek magasak és szépek. = The buildings are tall and pretty.
Only if you use a simple plural form without any measurement. If you say "the buildings are tall" it's "az épületek magasak" (plural for both the subject and the adjective), but when you mention or imply the amount of them then both will lose the plural form, as in "Those three buildings are red" will be "Az a három épület piros", (instead of "Az a három épületek pirosak") or "Many buildings are red" becomes "Sok épület piros".