Example 1
毎日勉強をしなくてはならない。
Which translates to
YOU must study every day.
Example 2
３時半までに、銀行に行かなくてはならない。
Which translates to
By 3:30, I have to go to the bank.
Both sentences do not include either you or I, so how do I know which is which?
Help is much appreciated!
Example 1 could be “I must study every day” or “they must study every day”, etc. as well, but that’s just how they chose to translate it. And Example 2 could be “By 3:30 you have to go to the bank.” It all depends on context. If you heard this sentence irl, you’d probably know if the speaker meant they needed to study every day or you needed to study.
Like meagstudies explained, it’s all down to implication based on the situation. In the first case, imagine two scenarios:
A student got his report card back from school and it’s all F’s. If he says, 毎日勉強をしなくてはならない, then the only way it makes sense is if he means “I” have to study every day. Now if his mother sees the report card and says the same thing, the only way it makes sense is if the mother is saying “YOU” need to study every day.
In cases where the subject of the sentence isn’t automatically obvious, it could be spelled out with a name or pronoun to make it clear.