Simple Sentences:
* The detective tried to ascertain the suspect's alibi.
* The scientist needed to ascertain the accuracy of the data.
* Before making a decision, it's important to ascertain all the facts.
More Complex Sentences:
* Ascertaining the cause of the fire was the first step in the investigation.
* The doctor worked tirelessly to ascertain the patient's condition.
* The journalist's job was to ascertain the truth behind the politician's claims.
Formal Sentences:
* Ascertaining the authenticity of the artifact required meticulous examination.
* The committee was tasked with ascertaining the feasibility of the project.
* We need to ascertain whether this plan is viable before proceeding.
Note: "Ascertain" is a formal word, so it's best used in writing or in formal speech. It's a more formal way of saying "find out" or "determine."