Why can some words such as 安宁 be either a noun or a adjective?
Many words are used as different word type for convenience
For example, when the noun 鑽 (drill) was coined, it was convenient to use it as a verb for 'to drill' as well; when we have the existing adjective 哀傷 as in 哀傷往事(sad past), it is convenient to use it as a noun as in 這份哀傷 (this sadness)
The adjectival marker 的 can turn a noun or a whole phrase into an adjective. Also, many nouns can be turned into adjectives by adding other particles. For example 攻擊(n): attack --&gt; 攻擊性 (offensive) | 戲劇 (n): drama --&gt; 戲劇性 (dramatic). And standard nouns can be used as adjectival nouns as long as it makes sense (it is still a noun, but functions as an adjective for another noun). See How to use nouns as adjectives?