納得する

That's a great question! While both 納得する and わかる relate to understanding, they have a key difference.

わかる (wakaru) means to simply 'understand' something intellectually. You grasp the information.

納得する (nattoku suru) goes a step further. It means you not only understand it, but you also 'accept it as reasonable,' 'are convinced by it,' or 'are satisfied with the explanation.' It often implies a more emotional or personal acceptance.

Think of it this way: You might わかる an explanation, but not necessarily 納得する it if you don't agree with it or find it illogical.

Yes, absolutely! If someone presents an opinion or an argument and you find it sound and agree with it, you can use 納得する. For example:

彼の意見に納得しました。(Kare no iken ni nattoku shimashita.)
Hint: I was convinced by his opinion.

Yes, 納得する is a perfectly polite and natural verb to use in various situations, both formal and informal. It doesn't carry any negative connotations.

You can use 納得する with the particle に (ni) to indicate what you are convinced by or what you accept.

Examples:
1. 先生の説明に納得した。(Sensei no setsumei ni nattoku shita.)
Hint: I was convinced by the teacher's explanation.

2. 彼の言い訳に納得できない。(Kare no iiwake ni nattoku dekinai.)
Hint: I can't accept his excuse (I'm not convinced by his excuse).

You can! If you've been struggling with a concept and then it suddenly 'clicks' and you feel satisfied with your understanding, 納得する fits well. It's more than just knowing the information; it's about accepting it as clear and reasonable.

For example:
難しい問題を考えて、やっと納得できた。(Muzukashii mondai o kangaete, yatto nattoku dekita.)
Hint: After thinking about the difficult problem, I finally understood/was convinced (by the solution).

Not really a 'more casual' single word, but 納得する itself is commonly used in casual conversation. The politeness comes more from the verb ending (e.g., 納得しました vs. 納得した).

Sometimes, people might just say 「なるほど」(naruhodo), which means 'I see' or 'That makes sense,' indicating understanding and acceptance, but it's not a direct verb equivalent.

It can imply agreement, but it's not a direct translation of 'to agree.' While agreeing often involves being convinced, 納得する focuses more on the feeling of being satisfied or finding something reasonable. You can agree with something without being fully 納得する, and vice-versa.

For example, you might agree to go somewhere with friends, but you might not be 納得する with the destination if you wanted to go somewhere else.

No, 納得する is typically used for a person's understanding, conviction, or acceptance of something. Inanimate objects don't 'understand' or 'get convinced.' You wouldn't say, for example, 「机が説明に納得した」(Tsukue ga setsumei ni nattoku shita - The desk was convinced by the explanation).

The negative form is 納得しない (nattoku shinai) in the plain form, and 納得しません (nattoku shimasen) in the polite form. It means 'not to be convinced,' 'not to accept as reasonable,' or 'not to understand satisfactorily.'

Example:
彼の言い訳に納得できなかった。(Kare no iiwake ni nattoku dekinakatta.)
Hint: I couldn't accept his excuse (I wasn't convinced by his excuse).

Yes, 納得する is a very common and useful word in daily Japanese conversation. You'll hear it frequently when people are discussing explanations, arguments, or trying to convey that they understand and accept something.