also: ひつ
noun
writing brush; paintbrush; pen
Refers to the physical tool for writing or painting, including traditional brushes and modern pens.
祖父は筆で手紙を書く。
My grandfather writes letters with a brush.
新しい筆を買った。
I bought a new paintbrush.
noun
brushwork; penmanship; something drawn with a brush
Refers to the act of writing or drawing with a brush, or the resulting work. Often used in artistic or calligraphic contexts.
彼の筆は力強い。
His brushwork is powerful.
この絵は彼女の筆によるものだ。
This painting is by her brush.
noun
writing; the written word
Only with reading ふで
Abstract use referring to the act of composing text or the resulting written work. Restricted to reading ふで.
彼は筆で生計を立てている。
He makes a living by writing.
その記事は彼の筆になるものだ。
That article is from his pen.
counter
Only with reading ひつ
Counter for lots or plots of land. Restricted to reading ひつ.
この土地は三筆に分かれている。
This land is divided into three lots.
The kanji 筆 has been used historically for writing brushes. The reading ふで is the native Japanese word, while ひつ is the Sino-Japanese reading used in compounds and the counter sense.