The simple past and the past perfect share some signal words. The table below explains the differences for three of the most common signal words.

Simple PastPast Perfectafter

The clause with after has no verb

Example:
After the safety demonstration, the plane took off.

The verb appears in the clause with after

Example:
After the flight attendants had completed the safety demonstration, the plane took off.
when

The action in the clause with when takes place after the action in the first clause.

Example:
She had already fastened her seatbelt when the flight attendants gave the safety demonstration.

The action in the clause with when was completed just before the new action began.

Example:
When she had fastened her seatbelt, the flight attendants gave the safety demonstration.
before

The action in the clause with before began after the other action was completed.

Example:
All passengers boarded/had boarded before the plane took off.

The action in the clause with before was not yet completed when the other action began.

Example:
The plane took off before all passengers had boarded.