English Grammar

What is the difference between "is going to do" and "is doing" ?
is going to do = will do in the future = He is going to do his homework in an hour.
is doing = He is doing his homework.

"When do we board the plane" or "When will we board the plane" ? (What is the difference?) Basically, these two sentences mean the same.
lst one is normally used in conversation while the second one is normally both in conversation or writing.

"The train leaves at 6 o'clock" or "The train is leaving at 6 o'clock" ?
These also means the same. lst one means the train leaves at 6 o'clock daily as the schedule or time table.
The 2nd one means the train will leave at 6 o'clock. or we can say the train is going to leave at 6 o'clock.

"John is leaving at 5 o'clock" or "John leaves at 5 o'clock" ? These two also have the same meaning. - see explanation for the train please

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"is going to do" means you intend to do sth
"is doing" : 1. you are proceed to do sth. it is a Present Tense
2. It is the same meaning as "is going to do"


"When do we board the plane"or "When will we board the plane" ?
Generally they are in the same meaning.


"The train leaves at 6 o'clock"
"John leaves at 5 o'clock"

They are simple tense, no need to make some detailed explanation.



"John is leaving at 5 o'clock."
"The train is leaving at 6 o'clock" ?

they are like "is doing sth" or "is going to do sth", future tense.
just to emphasize that "What Are You Doing at that time"

"is going to do" 將會做．．．
"is doing"我依家做緊

"When do we board the plane" 我地係幾時上機（過去）
"When will we board the plane"　我地將會幾時上機

"The train leaves at 6 o'clock"火車在６時開出（好informative)
"The train is leaving at 6 o'clock"火車會在６時開出（有催人快d的感覺）

"John is leaving at 5 o'clock" or "John leaves at 5 o'clock" ：同上