godan verb (-ku)
write in the square style; write in the standard script
Calligraphy term: writing characters in the formal, block-style script (kaisho). Used in contexts of brush writing or penmanship.
書道の授業では、まず真で書く練習をします。
In calligraphy class, we first practice writing in the square style.
楷書 is the noun for the square/standard script style itself, while 真で書く is the verb phrase meaning 'to write in that style'.
Compound of 真 (shin, 'true' or 'correct') + で (particle) + 書く (kaku, 'to write'), literally 'to write in the correct/formal manner'. The term is used in calligraphy to refer to the standard script.