“someone” in Japanese
We usually translate “dareka” as someone and “nanika” as something, but they are the examples which can’t be translated directly.
I’m busy tonight. Because I am going to meet someone.
How would you say this in Japanese?
こんやは いそがしい。だれかとあうから。(konya wa isogashii. dareka to au kara.)
Hmm, this translation is strange, We Japanese would say it like this.
こんやは いそがしい。ひとと あうから。(konya wa isogashii. hito to au kara.)
The word “dareka” is made from the question “dare desu ka”, therefore dareka implies that I don’t know who that person is. You can’t identify that person.
In other words, “dareka to au” means that you will meet someone you don’t know. It sounds stragne that you will meet a stranger, though you have an appointment. Even if they are a person you have never met like a blind date because you made an appointment with them, you have some information about this person and can identify them to some extent.
On the other hand, when you say “hito to au”, the speaker knows this person, but the listener doesn’t. So, the speaker just doesn’t mention the name of this person. Of course, it is possible to be more specific, such as “tomodachi to au” (I’m meeting my friend.), “ani to au” (I’m meeting my older brother.) or “shokai shite moratta hito to au” (I’m meeting a person to whom I was introduced.)
Here are the examples of dareka.
だれかが わたしのさいふを ひろってくれた。(dareka ga watashi no saifu o hirotte kureta.) Someone found my wallet.
（みちに まよったとき）だれかに みちを きこう。(michi ni mayotta toki, dareka ni michi o kikou.) (When I am lost) I will ask someone for directions.
In both cases you don’t have any information like their names. They are complete strangers.
Next, let’s think about nanika and something in the same way. For exampe, you would ask a host the following when you are invited to a house party.
Should I bring something? なにか もっていきましょうか。(nanika motte ikimashou ka?)
In this case, you don’t know what to bring and you cannot specify it. Therefore, you need to use “nanika” here.
What would you say when you give a friend a souvenir from your trip? In English, you would say, “I brought you something.” But, you can’t translate “something” as “nanika” here because the person who bought the souvenir knows what it is. You always know what you bought. Therefore, we say it like this in Japanese:
おみやげを もってきたよ。(omiyage o motte kita yo.) I brought you a souvenir.
いいものを もってきたよ。(ii mono o motte kita yo.) I brought you something nice.
Concerning this theme, more specific expressions are used in Japanese than in English.