KYOUMI GA ARU (興味がある)
To express interest that one has in something, the noun KYOUMI (興味) is used, which literally means “interest.” Then we add the particle GA to mark the topic, and at the end we add ARU (ある). In a spoken language, the particle GA can be omitted. The final structure is:
KYOUMI + GA + ARU
|(Boku wa) kyoumi ga aru.|
|I’m interested / It interests me.|
Literally: (In my case), an interest exists.
This sentence is grammatically correct and often used to express an interest in something. To identify what you are interested in, use the particle NI (に). The structure then becomes:
Element + NI + KYOUMI + GA + ARU
|Nihongo ni kyoumi ga aru yo.|
|I have an interest in the Japanese language / I’m interested in the Japanese language.|
Literally: In the Japanese language, an interest exists.
To express an interest in doing something, the verb must be nominalized using the particle KOTO (こと). Nominalizing simply means that a verb is turned into a noun. Then the same structure can be used.
Verb + KOTO + NI + KYOUMI + GA + ARU
|Hashiru koto ni kyoumi ga arimasu.|
|I’m interested in running.|
Literally: An interest in running exists.
There is another structure to do this: nominalizing with the particle NO (の) and adding a second particle, NI (に), to show that it’s the topic of the sentence, 走るのに興味があります.
Element + NI + KYOUMI + O + MOTSU
|Kabu ni kyoumi o motte imasu.|
|I have an interest in the stock market.|
You can also use the adjective FUKAI (深い) to express a strong personal interest. This adjective literally means “deep.”
|Nihon ni kyoumi ga fukai.|
|I’m much interested in Japan.|
Literally: An interest in Japan is deep.
|Tsuri ni fukai kyoumi ga arimasu.|
|I have a great interest in fishing.|
At last, to express the interest of someone else in Japanese, you should use phrases like these depending on the situations: it seems that, it appears that.
One important final note. You may already know the adjective OMOSHIROI (面白い). It also means “interesting,” but in a fun and entertaining way. For instance, if you visit a museum about the Second World War in Japan, avoid using 面白い...