Hello

A high school girl wrote a consolation letter to a solider because school said if she write a letter, she can get credit for her resume for college. So she wrote it but her letter was careless. The content of it had a slightly mocking tone rather consoling. Her letter has arised a controversial in our society.
I think if she had decided to write the letter to get credit, she should have written in a careful way.

I have a question.

I want to know if " if she had decided to write the letter to get credit, she should have written in a careful way." is correct.
I mean, she did something wrong in the past, and I want to express my feelings about what she did.
What I'm saying is that if she had decided to do something before doing something, she should have tried her best.
But I'm not sure the tense of my sentence "if she had decided to write".
Could you please correct and explain it to me?
Thank you for your time and your help.

Question

Hello

A high school girl wrote a consolation letter to a solider because school said if she write a letter, she can get credit for her resume for college. So she wrote it but her letter was careless. The content of it had a slightly mocking tone rather consoling. Her letter has arised a controversial in our society.
I think if she had decided to write the letter to get credit, she should have written in a careful way.

I have a question.

I want to know if " if she had decided to write the letter to get credit, she should have written in a careful way." is correct.
I mean, she did something wrong in the past, and I want to express my feelings about what she did.
What I'm saying is that if she had decided to do something before doing something, she should have tried her best.
But I'm not sure the tense of my sentence "if she had decided to write".
Could you please correct and explain it to me?
Thank you for your time and your help.

Answer

Pretty good, but could be even better. Your sentence, “I think if she had decided to write the letter to get credit, she should have written in a careful way” is an example of the 3rd conditional. If you’d like to improve it, it might be better to write it as a mixed conditional. Also, while your word choice is OK (correct and understandable), I’m going to suggest some changes. Here is your grammar, but more succinct, and with better vocabulary, more idiomatic phrasing.:
“I think if she wrote the letter for credit, she should have written it more carefully.”

Explanation of conditionals:
“If she wrote the letter for credit”: This is a past *real* conditional — you believe that she actually did write the letter for credit. That being the case….

“she should have written it more carefully”: This is the 3rd conditional, i.e. contrary-to-fact past. You present this alternate reality as a way of criticizing her (past) behavior.

Answer

I do understand this sentence. You  are using "careful" in an unusual way. May I suggest;
-she should have written in a more conscientious way.
or
-she should have written with more feeling.
or 
-she should have taken more care to write an honest letter.

Answer

I would say "because/seeing as she decided to write the letter to get credit, she should have written it in a more careful way."

Using "if she had..." means that she didn't - your are just hypothesising. Because the letter was actually written, this is not correct.

Hopefully this is what you meant.