Dear Ms.Sue, I am very grateful for your explanation. Please have a look at one more sentence.
"They had been trying for more than ten years to conceive a baby naturally but finally had to resort to in vitro fertilization".
Is the word order OK? Do you think it's possible to use "tried"?
Thank you very, very much.
Yes. That's an excellent sentence.
Either tried or had tried or had been trying are correct.