I woke up this morning constructing sentences in my head in Kreyol. One of them was "Yon vreman gran machin." and "Vreman gran machin nan." What I was wondering is on this last sentence could or should it be "Machin nan vreman gran. Mesi. Jan Rachal

You can pretty much use all three examples in a sentence.  They are all different.
The first two examples aren't complete sentences,but they definitely can be incorporated in a sentence.
The third sentence is complete - correct - and anfòm.

As for the word 'gran', it does translate great, awesome, wonderful, mighty, etc....
You might instead use the word 'bon' for "reliable and great" car; and the word 'gwo' for "big size" car; and 'gran' for a reputable brand of a car for example: "Toyota se gran mak machin."

Dakò, here are your examples:

#1 - "Yon vrèman gran machin...." - "a really great car...."
        Did you mean "Sa se vrèman yon gran machin" - "This is truly a 'great' car"?

#2 - "Vrèman gran machin nan..." - "The truly great car ......"

#3 - "Machin nan vrèman gran." - The car is really 'great'.

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