The sentence 'was very diligent' is correct and can be used in written English.
You could use it in a sentence such as, "He was very diligent in his work, and the results showed."
"But he was very diligent.
He was very diligent and passionate about his work".
"He was very diligent with everything he did and how he did it," Shelton said.
"The Australian Rugby Union was very diligent about the way it went about the process.
The department's Web site lists her as a firearms instructor, and Captain Sudol said Officer Collura was "very diligent in everything she did".
Co-workers said Ms. Wilson, 48, an assistant vice president at Aon, always tried to make people feel valued and was very diligent.
"I'm very diligent about that," he says.
"She's very diligent, divisive and decisive, and that's all I need to think about.
But they were very diligent in apprising me about the options".
You have to be very diligent as a parent and as an adult.
"His mother and father were very diligent about attending," Mr. Maurer said.
