One of my Asian students asked me to explain the difference between phrases
A train will leave at 8:00.
A train leaves at 8:00.
To tell the truth I am not sure what would be a good explanation for the asian person. From what I know, they do not have past or future times. Please, help!

I generally start tense explanation with kids 10+. The older the students the easier it is to explain since they will have had some grammar explanation in their L1. It really depends on how old they are, but the way I explain tenses is to draw a time line.

____________ now
_____________|_ _ _ _ _ _ _ _ _ _ _ _ >

There are certain events on the time line, and they all have to do with a time reference point. In your example: A train will leave at 8:00 (tomorrow.)

So, we can place that on the time line in the future. This sentence is talking about a specific train at a specific time point in the future.

In your example the single sentences is extracted for grammar practice. If it were in a conversation the point in time would be clear - tomorrow or next Tuesday when you are planning to go somewhere. So, when explaining the tense, you can add that in parenthesis to show that this exact time reference is in there it has just been eliminated as it would be understood in the conversation.

Present Simple on the other hand is completely removed from the time line. I generally place it well above the line and explain that "time doesn't matter" (really meaning the time line doesn't matter.) So, we're not talking about a specific train. We're expressing a general truth or present fact: A train leaves at 8:00 (every day/on Mondays/etc.)

________present truths: My name is Mark. This chair is black. Trains leave at 8:00, 9:00 and 12:30...
____________ ^


____________ now
_____________|_ _ _ _ _ _ _ _ _ _ _ _ >

Thank you, Mark!

My student is not a child, actually, she is a Vietnamese English teacher in her 40s . I believe she was looking for a more detailed and reasonable explanation about particular situations in which we would prefer PS or PF tenses.

Could you give some more examples, please? Thank you.

For an adult, I would use the same explanation with the time line. The Present Simple is an expression of facts that are true at the present time.
I don't have a fish.
My sister likes soccer.
He doesn't have any brothers or sisters.
I don't play basketball.
A man plays golf on this course (every day.)

If the situation changes, that change would be in the past. Then, we are talking about a situation in a definable period in the past.
I didn't have a fish last year. However, now, I have 3 fish.
My sister liked soccer when she was a child, but now she likes tennis.
A man used to play golf on this course every day, but he was injured. He stays home most afternoons.

The Future Simple is used when you are talking about a specific event at a certain time in the future. (It could be an undetermined time in the future with sentences like 'If I find your bag, I will call you.' but no need to confuse the situation. This would still be a specific time - when I find your bag, just not defined by tomorrow, next week, on Tuesday, etc.)

I will get a new fish next week.
My sister will play soccer (on Monday.)
I won't play basketball in the next tournament.
A man will play golf on this course (tomorrow.)

Both your sentences can be true and used. It's just a matter when you are talking about or if you are talking about 'when' at all.

A train leaves at 8:00.
- (every day,) there is a train that leaves at 8:00. This is a regular occurrence and we're not talking about any specific date. This is a general truth. The schedules says one train is scheduled to leave at 8:00 (every day.)

A train will leave at 8:00.
- (tomorrow morning) there is a train that will leave at 8:00. This is talking about a specific point in time, tomorrow at 8:00am. This is a point in the future and therefore we use the future simple.

The confusion is probably with the example. Either could be used in a conversation like this:
A: I have to be at my meeting by 10:30. Can you check the train schedule?
B: Sure. (Checks schedule) Trains leave at 8:00 and 9:00.
A: I better take the 8:00 train. I don't want to be late.

- the information is just a general truth - the train schedule


A: I have to be at my meeting by 10:30. Can you check the train schedule?
B: Sure. (Checks schedule) Trains will leave at 8:00 and 9:00.
A: I better take the 8:00 train. I don't want to be late.

- the information is expressing actions by tomorrow morning's trains.


With these 2 sentences, the regular occurrence or tomorrow's specific departure don't change the overall meaning of the information the speaker is trying to get across. So, in this example the sentences are interchangeable, in my opinion. So in this case, there is no reason to choose one over the other. You need a different example if you want to explain when we would actually choose the PS over the FS.

There are many instances where different forms are interchangeable. This generally happens with simple or very common sentences. If she doesn't understand the difference in PS and FS then I would suggest different sample sentences:
I play golf.
I will play golf tomorrow.

After rereading what I wrote, I think I want to change my answer a little

Of my 2 examples, I would probably choose the present simple:

A: I have to be at my meeting by 10:30. Can you check the train schedule?
B: Sure. (Checks schedule) Trains leave at 8:00 and 9:00.
A: I better take the 8:00 train. I don't want to be late.

The reason is Speaker A has defined the context of the situation with the present simple "I have to be at my meeting by 10:30." So, the conversation as it stands is just talking about present truths.

A different example:
A: I will need to be at my meeting by 10:30. Can you check the train schedule?
B: Sure. (Checks schedule) Trains will leave at 8:00 and 9:00.
A: I better take the 8:00 train. I don't want to be late.

In this case, Speaker A has defined the context of the situation as tomorrow morning. Then we're talking about a specific time in the future and we may be more inclined to use the Future Tense.

However, I still would call them interchangeable. I wouldn't find either response in either situation unnatural. There's just not enough context or the information is clearly understandable with either expression.

Thank you very much, Mark