But in Spanish, information about the subject is left in the sentence. "Lo leí" cannot have any subject but me, and "caminaba por la calle" could only be me, h

But in Spanish, information about the subject is left in the sentence. "Lo leí" cannot have any subject but me, and "caminaba por la calle" could only be me, him, or her. In Japanese, the information about the subject is substantially more indirect. For example, in "昨日サボったわよ" the subject could be almost anyone, but the speaker is a woman and she is probably talking to a friend or family member. You're on track with the "gramatically empty placeholder". We have expletive pronouns, like "it" in "it is raining", which one could consider to only serve grammatical purpose—it is only in the sentence precisely because the sentence must have a grammatical subject, not because the sentence has a subject in any semantic sense. There is some disagreement about this, however.