Here is the question: your boss is going to Osaka for a conference next Tuesday and he asks you to come to help him. How would you answer in Japanese?
It can neither be 私は行きます (watashi wa ikimasu) nor 私が行きます (watashi ga ikimasu). 私は行きます (watashi wa ikimasu) sounds like "I don’t know what other people will do, but I will go." 私が行きます (watashi ga ikimasu) sounds like "I don’t want other people to go, but I will go and take all the credit."
So, what’s the best way to answer? You should simply answer, 行きます (ikimasu). 行きます (ikimasu) is not an abbreviation of 私は行きます (watashi wa ikimasu) or 私が行きます (watashi ga ikimasu), which is different from English expressions when you omit the subject such as “see you tomorrow.”
A linguist says that Japanese doesn’t have nouns, but rather consists of predicates. That makes sense right? We often omit the subject and particles, and we are attuned to one another when we communicate. I think there’s a sense of understanding, and sometimes it doesn’t even require words.
Kinou wa totemo tsukarete itanode, uchi ni tsuku to sugu ni nemashita. "Was exhausted yesterday, so went to bed right after got home."
Let’s put the subject in the sentence.
昨日は私はとても疲れていたので、私は家に着くとすぐに私は寝ました。Kinou wa watashi wa totemo tsukarete itanode, watashi wa uchi ni tsuku to watashi wa sugu ni nemashita. "I was exhausted yesterday, so I went to bed right after I got home."
As a native speaker of English, you may think the second sentence sounds more natural, but to a native Japanese speaker, it sounds really annoying and wordy. First of all, we don’t put the subject in every sentence, especially when the subject is “I” or “you.” But in English, you put the same subject in everywhere, over and over like “I” in the second sentence. It’s because English grammar requires a subject for every part in the sentence. But in Japanese, this is not the case. English even uses a pronoun to avoid repeating the same proper noun, but in Japanese, it’s deemed unnecessary and removed entirely.
In Japanese, you can make a complete sentence without the subject, so you don’t have to keep using it throughout the sentence. Secondly, if you use the subject even though you didn’t need to, it sometimes changes the meaning from what you are trying to say. As I explained above, 私はとても疲れていた (watashi wa totemo tsukarete ita) means “I was exhausted,” and sounds like only you are tired but not other colleagues. If it’s about just you, you can add “I,” but if you are not really trying to mention other people, you can’t use it.
私は新しい携帯が欲しいです。Watashi wa atarashii keitai ga hoshii desu. "I want a new cellphone."
私は趣味が多いです。Watashi wa shumi ga ooi desu. "I have a lot of hobbies."
私はウイスキーが嫌いです。Watashi wa whisky ga kirai desu. "I don’t like whisky."
In these three examples, it’s obvious that you’re not trying to directly compare yourself to others, and it’s a story about “you” so it’s okay.
On the other hand, you cannot say:
A: 昨日は残業しましたか？Kinou wa zangyou shimashita ka? "Did you work overtime yesterday?"
B: いいえ、私はしていません。Iie, watashi wa shiteimasen. "No, I didn’t (but I don’t know about the others)."
A: 宿題終わりましたか？Shukudai owarimashita ka? "Have you done your homework yet?"
B: はい、私は終わりました。Hai, watashi wa owarimashita. "Yes, I have (I don’t know about everyone else though)."
A: パーティーはどうでしたか？Party wa dou deshitaka? How was the party?
B: 私は楽しかったです。Watashi wa tanoshikatta desu. "I had a good time (but I think everyone else hated it)."
In these three examples, there are other people involved in the story, so it sounds like you’re directly comparing yourself to them if you use the subject.
Also, you don’t have to say the subject when people understand who you are talking about.
鈴木さんに会ったら、楽しそうだった。Suzuki san ni attara, tanoshisou data. "When I met Mr. Suzuki, (he) looked like he was having fun."
鈴木さんが田中さんに会った時、楽しそうだった。Suzuki san ga Tanaka san ni atta toki, tanoshisou data. "When Mr. Suzuki met Mr. Tanaka, (he) looked like he was having fun."
Who looked like they were having fun? The first sentence says that Mr. Suzuki looked like he was having fun. Also, the second sentence means Mr. Tanaka looked like he was having fun. In English, we need a pronoun (he), but in both of these Japanese sentences, the pronoun is omitted for 楽しそうだった (tanoshisou data), which means “looked like having fun.” But we can still understand who they are talking about from the context.
母が帰ってきたので、電話を切った。Haha ga kaettekita node, denwa o kitta. "(I) hung up the phone because my mother came back home."
Who hung up the phone? “I” did.
ニックが日本に来たら、お好み焼きを食べよう。Nick ga nihon ni kitara, okonomiyaki o tabeyou. "When Nick gets to Japan, let’s eat okonomiyaki."
Who’s going to eat okonomiyaki? “Nick and I”.
When you are not sure if you need the subject or not, it would probably be better not to use it. If people don’t understand and ask you who you are talking about, you can add it. That will be no problem because we omit the subject more often than not. Also, not only when you speak, but you can omit it when you write as well.
I think omitting the subject shows a big part of Japanese culture in which we try to read inferred meanings from short or vague sentences. This is a very unique and interesting characteristic of the Japanese language. It may be tough at first, but you will get it with practice, so keep studying!