"The wind blows strongly."
What about the adjective form then? I mean if the sentence is negative shouldn't it then also be つよくないです。
Yes but on the other hand, typing using googles Japanese input keyboard on a phone is a pain in the butt compared to the standard keyboard... I feel like thats a point in the pro romanji column... Lol
Try out Swiftkey's Japanese mode. You will have to use their suggestion bar if you want to type katakana or kanji though.
So, why do we do the negative form of -i adjectives with -ku, but here tsuyoku its "strongly" instead of "not strong"?
No; we do the negative form with -kunai. The -ku form is the adverb form, hence "blows strongly" (strongly, being an adverb, modifies the verb), instead of "the wind is strong" or "the strong wind blows".