よく勉強する

You can use this phrase very simply. Just put よく right before the verb 勉強する. If you want to be polite, use 勉強します. To talk about the past, use 勉強しました. It works for yourself or when talking about others. "My sister studies hard" becomes 妹はよく勉強する. It sounds natural and very encouraging. You can also use it to describe a general character trait.

Use it at school when talking about classmates. It is great for parents praising their children. You can use it in a job interview too. Tell them you "studied hard" to gain a specific skill. It shows you are a reliable and hardworking person. Text it to a friend who is stuck in the library. It is a great way to acknowledge someone's grind.

Don't use it for passive activities. Watching a movie isn't "studying hard" usually. Avoid it if someone is just doing the bare minimum. It implies a level of intensity and focus. If you just checked a dictionary once, don't use it. It might sound sarcastic if the person is clearly lazy. Also, don't confuse it with just "liking" a subject. You can like math without studying it hard.

Japan has a deep respect for the "effort culture." The word ganbaru (to do one's best) is everywhere. よく勉強する is a specific way to show that effort. Students often stay at school until late evening. Being called a hard worker is a high honor. It shows you have discipline and respect for knowledge. In Japan, the process of trying is often as valued as the result.

You might hear よく勉強したね from a teacher. This adds a warm "didn't you?" at the end. もっと勉強してください means "please study more." 勉強家 (benkyouka) is a noun for a "hard worker." よく勉強している means someone is currently studying hard these days. Each variation keeps that core feeling of dedication. You can also use 一生懸命 (isshoukenmei) for even more intensity.