The phrase "ascertain why" is correct and usable in written English.
You can use it when you want to find out the reason or cause behind something.
Example: "The manager asked the team to ascertain why the project was falling behind schedule."
Alternatives: "determine the reason" or "find out why".
Officers had been sent there to ascertain why mullahs were preaching against the British.
To ascertain why people attend hospital emergency departments (ED) for low complexity health problems.
The researchers were trying to ascertain why certain antibiotics work the way they do.
Use phrases like "what would that mean to you?" or "what would happen if you didn't get it?" to ascertain why it matters.
Researchers have managed to successfully remove all of the protruding eels, all from juvenile seals, but are still trying to ascertain why this is happening.
He is to meet his executive team this week to ascertain why the force has blown its overtime budget for 2018 by almost €15m.
With The Hateful Eight about to arrive in cinemas, you could almost mistake it for a timely proposition but, beyond the title, it's near impossible to ascertain why it's been made, or for whom.
As part of the effort to ascertain why young drivers are more at risk for car crashes, attention has recently turned to the effects of family, including the intergenerational transmission of driving styles from parents to offspring.
The motives of people for visiting urban green spaces (UGS) mirror their needs and expectations on UGS, which help to ascertain why people patronize UGS in the way they do.
But Dr. Wimmer is not resting on his synthetic laurels: he and the team are already back in the lab to ascertain why their virus is weaker than the natural stuff.
Mr. Méndez said that he spent 10 minutes with the Barnett representative trying to ascertain why the Ryvor workers were being let go while workers already on Barnett's payroll, some of whom he believed were also undocumented, were untouched.
