Learn Hardcore Japanese: kono kanzi ha kakinikui desu.

書きにくい means “difficult to write” or “hard to write.”
It’s formed by taking the verb stem 書き (from 書く, “to write”) and adding the suffix にくい, which creates an i-adjective expressing that an action is hard to do.

What part of speech is 書きにくい?

書きにくい is an i-adjective (形容詞). I-adjectives end in い in their plain form and can directly modify nouns or be used predicatively with です.

Why is there no を before 書きにくい? Shouldn’t 漢字 be marked as the direct object of write?

In this sentence 書きにくい is an adjective, not the verb 書く. The structure is:
Topic (この漢字) + は + i-adjective (書きにくい) + です.
Since it’s an adjective phrase (“hard to write”), you don’t use the object marker を here.

What is the function of は in this sentence?

The particle は marks the topic. この漢字は means “As for this kanji…” It tells the listener what you are talking about.

Could you use が instead of は, and what difference would it make?

Yes: この漢字が書きにくいです is grammatically correct.
• With は, you make a general statement about the kanji.
• With が, you emphasize that this particular kanji (and not others) is hard to write.

Why is です used at the end of the sentence?

です is the polite copula, making the statement polite. Here it functions like “is,” linking the adjective to the topic in polite speech. Without です, the sentence becomes more casual.

How would you say this informally?

In casual speech you drop です: この漢字は書きにくい.
You might even use a colloquial ending: この漢字、書きにくいね.

What is the opposite of 書きにくい (“hard to write”)?

The opposite is 書きやすい, formed by verb stem 書き + suffix やすい (“easy to ~”). So 書きやすい means “easy to write.”

Are there synonyms like 書きづらい, and is there any nuance difference?

Yes, 書きづらい also means “hard to write.” Both ～にくい and ～づらい turn verb stems into adjectives of difficulty.
• にくい often indicates physical or intrinsic difficulty.
• づらい can add a nuance of psychological discomfort or awkwardness.
In most cases for physical difficulty they’re interchangeable.