Subjects and Deletions

Usually, you don't have to worry about whether to use は or が because most subjects can be deleted. "You can't get something wrong if you left it out in the first place." So we'll look at the parts of sentences that you can delete, starting with subjects.

If you turn to a Japanese person and suddenly make a statement:

then the listener will assume that the subject is you. So don't bother supplying a subject. To do so is in fact unnatural; Japanese people don't supply subjects in conversation if they're obvious.

If you turn to a Japanese person and ask them a question:

The listener will assume the subject is himself or herself. Easy! Most one-on-one conversations where either you or the listener is the subject don't need an explicit subject. So there's no chance of screwing up は and が.

If you want to make a statement or ask a question about some other person, use は after that person's name or title the first time you mention them:

Here the は introduces a change of subject. In this example it signals a change from the default 'you the listener' to the 社長. After you establish that you're talking about the president you can go back to dropping subjects again: Don't be too forward making assumptions about other people. This prevents the listener from thinking the question is back to being about themself. There's a strong tendency for questions to erase understood info and you have to signal that things are unchanged. Usually you play with the verb a little bit to get this across. Notice that the change in time being talked about was also signalled with a は.

This tendency to delete subjects in Japanese parallels the behavior of an English native using simple pronouns such as, 'I', 'you', 'he', 'she', and 'they'. When you comment about yourself you use 'I' (in Japanese, delete). When you ask about the listener you use 'you' (in Japanese, delete). When you've first established someone and then continue discussing that person, you use 'he' or 'she' (in Japanese, delete). See? It's simple. In linguistics these are called anaphors, verbal markers which refer to previously established topics. In English (and in most other Indo-European languages) we use simple pronouns as anaphors. In Japanese explicit anaphors are not used, instead the anaphoric position in a sentence is simply left empty.