arienh4:
Ŝi: Strange, li diris nenion al mi.
Isn't strange stranga? As strange would be an adverb, and you're talking about an adjective right?
Don't mean to correct you, I'm still learning

Not at all, that's a very good question. If we are describing a specific thing, as in 'The room is strange' we do indeed use the adjective and say La ĉambro estas stranga. But when we say 'It's cold' or 'That's strange' there is no such well-defined thing, which 'It' or 'That' might refer to, so we are really describing the verb 'to be' (whose role in such a situation is called an impersonal verb), hence we use an adverb (a word that tells us about a verb). For a reference, see section 5.1, foot of page 18 of the detailed grammar on this site, and the notes at the end of lesson 10 of Jen Nia Mondo 1.

Miland:

arienh4:
Ŝi: Strange, li diris nenion al mi.
Isn't strange stranga? As strange would be an adverb, and you're talking about an adjective right?
Don't mean to correct you, I'm still learning

Not at all, that's a very good question. If we are describing a specific thing, as in 'The room is strange' we do indeed use the adjective and say La ĉambro estas stranga. But when we say 'It's cold' or 'That's strange' there is no such well-defined thing, which 'It' or 'That' might refer to, so we are really describing the verb 'to be' (whose role in such a situation is called an impersonal verb), hence we use an adverb (a word that tells us about a verb). For a reference, see section 5.1, foot of page 18 of the detailed grammar on this site, and the notes at the end of lesson 10 of Jen Nia Mondo 1.

You're right, I forgot that strange could be an adverb too.