Questions & Answers about Den här boken är tung.
As a predicate adjective (one that follows är), you use the base form. Since bok is a singular common-gender noun (an en-word), the adjective stays tung.
• tungt would be for neuter (ett-words) or as an adverb
• tunga is the plural form
You shift the adjective into an attributive position (before the noun) and apply the definite form:
den tunga boken
Here den is the definite article for en-words, tunga is the definite singular adjective form, and boken is the definite noun.
Den här = “this” (something close to the speaker)
Den där = “that” (something farther away or previously mentioned)
bok is a common-gender noun (an en-word). That determines:
• Which demonstrative you use (den här)
• Which article (en bok vs. ett hus)
• Certain adjective endings in attributive and predicative positions.