Is there a sentence order?
그 차가 앞에 진짜 느렸어 is understandable but not near native or common either.
I was trying to say : The car in front is really slow
But an intermediate learned told me that it is unnatural and suggested
"앞에 있었던 차가 진짜 느렸어" instead.

I tried to follow the "Who when where what why/how [does]" but it doesn't seem to work or is it used for essays, articles etc

Is there a sentence order or does it depend entirely on the situation?

Question

Is there a sentence order?
그 차가 앞에 진짜 느렸어 is understandable but not near native or common either.
I was trying to say : The car in front is really slow
But an intermediate learned told me that it is unnatural and suggested
"앞에 있었던  차가 진짜 느렸어" instead.

I tried to follow the "Who when where what why/how [does]" but it doesn't seem to work or is it used for essays, articles etc

Is there a sentence order or does it depend entirely on the situation?

Answer

"The car in front is really slow" => (저) 앞차가 진짜/너무 느려.
"The car in front was really slow" => 앞차가 진짜/너무 느렸어. (앞에 있는 차 and 앞에 있던 차 are usually shortened to 앞차 - there's no need to say 앞에 있었던)

The only thing I can say about adverb placement order is that when(time) generally comes before where(place) in simple sentences (언제 어디서 이렇게 됐어?).   I'd say there's not much point in paying too much attention on such things - there are few clear rules and the rules are too easily broken by special expressions.  Just practice a lot with well written sentences.