In English, the question "What will you do tomorrow?" is grammatically correct and can be used, but it is more common to ask "What are you doing tomorrow?" because it is in the present continuous tense, which is used to talk about plans or arrangements for the future. Using the present continuous to talk about future plans is a common way to express that something has been scheduled or agreed upon, even though it hasn't happened yet.
It's a way of talking about future events that have been set, and it's considered more polite or formal than asking "What will you do tomorrow?" which implies that the person may not have any plans.
So, "What are you doing tomorrow?" is considered more appropriate in situations where you want to ask about someone's plans in a polite or formal way, or when you want to suggest a plan or schedule something with them.
So when you were asked what you are doing tomorrow, your answer would be like:
"I will go to Shibuya to play with my girlfriend."
"I am going to hang out with my girlfriend".
"I am studying for my exam tonight."
"She is cooking dinner for her family."
"They are playing soccer in the park."
"He is working on a project for his company."
"We are planning a trip for next month."
It can sometimes be difficult to distinguish between the present continuous tense being used to talk about a current happening or a future plan. However, there are a few ways to help determine the intended meaning:
Context: The context of the sentence can often provide clues about whether the present continuous tense is being used to describe a current happening or a future plan. For example, if the sentence is part of a conversation about someone's schedule for the day, it's more likely that the present continuous tense is being used to describe a current happening.
Time words and phrases: Words and phrases that indicate the time frame, such as "tonight," "next month," "this week," etc. can help determine whether the present continuous tense is being used to describe a current happening or a future plan.
Intonation and stress: The way the sentence is spoken can also indicate whether the present continuous tense is being used to describe a current happening or a future plan. For example, if the speaker emphasizes the word "am" or "is" it can indicate that the present continuous tense is being used to describe a current happening. If the speaker emphasizes the word "planning" it can indicate that the present continuous tense is being used to describe a future plan.