"We were not able to get a hotel room because we had not booked in advance."

Here's what I know, the Past Perfect is used when you want to expresses the idea that something occurred before another action in the past. But I read somewhere that says you can still use Simple Past instead if the sentence contains the word before or after because they already indicate which happens first such as the following ones.

1."She visited her Japanese relatives once in 2003 before she moved in with them in 2006."

2."He repaired many cars before he received his mechanic's license"

3."He received the grand prize after he won the competition"

Are they really grammatically acceptable?

If so, do native speakers commonly use Simple Past to indicate which one happened first when a sentence contains either before or after in informal conversations?

Thank you
What you have "read" is correct. Still, there are native speakers who use the past perfect in these cases.
GuyperAre they really grammatically acceptable?
Yes.

Guyperdo native speakers commonly use Simple Past to indicate which one happened first when a sentence contains either before or after in informal conversations?
Yes. It's especially common when both clauses refer to events that occurred as part of a common situation. Take your third sentence for an example. Winning a competition and receiving a prize are so closely connected and the order of these events is so obvious that the past perfect is completely unnecessary. Who do you know who ever received the prize beforewinning the competition? Emotion: smile

CJ
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"...the Past Perfect is used when you want to expresses the idea that something occurred before another action in the past."

Yes...when two events are not written in the correct chronological sequence, as in your 'hotel' sentence!

1."She visited her Japanese relatives once in 2003 before she moved in with them in 2006."

First she visited, then she moved in - the two events are in their correct time sequence.

The sentence might be better written as:

She visited her Japanese relatives once, in 2003, before moving in with them in 2006."

2."He repaired many cars before he received his mechanic's licence"

Again, the two events are in their correct time sequence.

The sentence might be better written as:
"He repaired many a car before receiving his mechanic's licence."

3."He received the grand prize after he won the competition"

This sentence is not clear: it makes it sound as if these were two distinct events, when they are not.

"He won Grand Prize in the competition."

Try:

"He was able to obtain employment after he learned how to handle himself in the interview situation."

Yes - here 'after' serves to preserve the time sequence of events. The use of the Past Perfect would emphasize the importance of what he 'learned'. e.g.

"He was able to obtain a much higher salaried position after we had coached him in job interview strategies."

Next: when and why would the Past Perfect by used as the first clause in a sentence/paragraph!Emotion: smileEmotion: geeked