"Become" with Nouns and Adjectives

There's a specific grammar structure used in Japanese to say that something has become [insert noun or adjective]. The structure is divided between nouns/na-adjectives and i-adjectives.

I-adjectives

• Drop the last い and add く
• 早い -> 早く
• 正しい -> 正しく

Nouns and na-adjectives

• Add に at the end of the word
• きれい -> きれいに
• 夏 -> 夏に

After that, the verb なる (which means "to become") is added, often in past tense. This is because people are usually saying "X has become Y", which translates to positive past tense.

Genki's first example sentence makes this grammatical structure more understandable.

日本語の勉強が楽しくなりました。
Studying Japanese is fun now. (Studying Japanese has become fun.)

So essentially, using this structure describes a change that has occurred. It may be relative or absolute, as the below example demonstrates.

温かくなりました。
It's become warm/warmer.

Without context, it's difficult to tell whether this sentence is saying that it's become warmer (but is still cold), or if it's become warm (and is no longer cold). This is why context is key.

リーさんはかわいくなった。
Lee-san's gotten cute. (He wasn't before.)

ジムは会社員になりました。
Jim's become a company employee.

スバルの料理はおいしくなった。
Subaru's cooking is really tasty now.

トムは変人になった。
Tom became a real oddball.

アリスの犬は大人しくなりました。
Alice's dog has become obedient.