To ascertain something means to find it out for certain, to discover the facts through careful investigation or inquiry. When a detective ascertains who committed a crime, she gathers evidence and pieces together the truth until she knows for sure, moving beyond guesses or rumors to establish what actually happened.

A teacher might ascertain whether students understand a lesson by giving them a quiz. A doctor ascertains what's wrong with a patient by running tests and asking questions. Before a principal decides on consequences for a playground incident, she needs to ascertain exactly what happened by talking to witnesses and examining the evidence.

The word suggests active effort and thoroughness. You ascertain something by figuring it out through deliberate investigation, using evidence and inquiry rather than hoping or assuming. A scientist ascertains whether her hypothesis is correct by conducting careful experiments. A historian ascertains facts about the past by studying documents, artifacts, and other reliable sources.

When you need to ascertain the truth about something, you're committed to finding out what really happened, not just accepting the first explanation you hear. It's the difference between wondering if your friend is upset and actually asking questions until you understand what's bothering them.