Note
Er ist besonders. - He is special.
Er ist besonders unglücklich. - He is especially unhappy.
Er ist heute besonders gut gelaunt. - He is in a particularly good mood today.
Der Kaffee ist heute besonders stark. - The coffee is particularly strong today.
as an adjective it declines. Let's for now remember the nom sg form besondere
der besondere Mann - the special man
die besondere Frau - the special woman
das besondere Kind - the special child
Usages of besonders
nichts Besonderes
nothing special
Here Besonderes is a nominalized adjective
In other words, it's the adjective besondere turned into a noun
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