Choose what the focus is on: the finished action (result) or the duration (process)

She had been writing for a long time because it was difficult for her to write an essay.

• Finished action (result)
• Duration (process)

She had written twenty stories before she became famous.

• Finished action (result)
• Duration (process)

I had ridden a horse before, so I knew how to do that.

• Finished action (result)
• Duration (process)

I had been riding my horse all day.

• Finished action (result)
• Duration (process)

Everybody said Tim was a great cook. He had cooked a delicious cake.

• Finished action (result)
• Duration (process)

Mary had been cooking for the party since early morning, and there was still a lot of work to do.

• Finished action (result)
• Duration (process)

I had been reading for three hours, and my eyes needed a break.

• Finished action (result)
• Duration (process)

Jim knew about the accident because he had read about it in the newspapers.

• Finished action (result)
• Duration (process)

How long had she been travelling when you met her?

• Finished action (result)
• Duration (process)

How many countries had she visited before she came to India?

• Finished action (result)
• Duration (process)