The Subject Has To Be The Same
“There are no ordinary moments.” – Dan Millman
You just learned that the ながら verb is the secondary action in a sentence. The other strange “rule” regarding ながら is that “the subject of the sentence has to be the same.”
What I mean by this is two different things can’t be doing something at the same time (and use ながら).
Let’s look at some correct sentences:
a わたし は おんがく を ききながら にほんご を べんきょうした。
While listening to music I studied Japanese.
a わたし は マクドナルドで はたらきながら だいがく に 行きます。
While working at McDonalds I will go to school.
These are both correct sentences. In the first one, the subject (you) stayed the same. For the second one, the subject also stayed the same (the sentence was also about you). Problems arise, however, when you try to talk about two subjects at the same time using ながら. For example:
a ともだち は マクドナルド で はたらきながら わたし は ハンバーガー を たべた。
While my friend works at McDonalds I ate a hamburger.
While this seems like it ought to work, it’s not grammatically sound. In order to do something like this you’d want to use the grammar 間に (あいだに), but to do that you need to learn v.te form (coming soon, but not quite yet!).
For now, just know that ながら can’t deal with multiple subjects.