Several parts combine into one character.

勉 is a phonetic-semantic compound: 免 (mian, "to escape / be exempt from") provides the sound, while 力 (power, exertion) provides the meaning. The picture is muscular effort applied to escape something difficult — "to push through, to exert oneself." From this root the character became the everyday word for studying hard.

Korean reading "myeon." Restricted to formal vocabulary: 勤勉 (geunmyeon, diligence), 勉學 (myeonhak, devotion to study), 勉勵 (myeollyeo, encouragement). The everyday Korean word for "study" is (gongbu-hada) — Korean does NOT use 勉強 as a study compound.

Mandarin miǎn, 3rd tone — and here is one of the most famous false friends in CJK. The compound 勉强 (miǎnqiǎng) means "reluctantly / with difficulty / to force oneself" in modern Chinese. "我勉强吃了一点" = "I forced myself to eat a little." The Chinese verb for "to study" is 学习 (xuéxí), never 勉强. Mainland speakers visiting Japan are often startled to see "勉強中" signs in classrooms.

Japanese on-reading ベン powers the single most important compound for any Japanese learner: 勉強 (benkyō, study). 勉強する is the verb every student says daily. Also 勤勉 (kinben, diligence). Kun-reading つとめる (tsutomeru) means "to strive." The semantic leap from "exertion" to "study" is uniquely Japanese — neither Chinese nor Korean made this jump.

Memory aid: 免 (escape) + 力 (power) — pushing with all your strength to escape ignorance. That's study.

In Japan·勉強 = study / learning (Japanese-only compound)

In China·勉强 = reluctantly / forced (in Chinese)