Learn French: J'étais au lycée il y a dix ans. - I was in high school ten years ago.

Normally, you find the imparfait (imperfect) stem by taking the present tense nous form and dropping the -ons. But être is the only verb in French that breaks this rule! Its stem is ét-. Here, we add the standard je ending (-ais) to get j'étais (I was).

Doesn't il y a mean "there is"? Why does it mean "ago" here?

You're right that il y a usually means "there is" or "there are." However, when you put a time period right after it—like dix ans (ten years)—it translates to "ago." So, il y a dix ans simply means "ten years ago."

Why is "high school" translated as lycée?

French uses very specific words for different school levels. Une école is an elementary school, un collège is a middle school (be careful, it's a false friend and doesn't mean university!), and un lycée is a high school.

Why do we use au instead of just à for "in"?

In French, you are essentially saying "in the high school." Because lycée is a masculine noun, it takes the article le. The preposition à (at/in) combined with le must always contract into au.

Is there a trick to pronouncing dix ans?

Yes! Normally the "x" at the end of dix sounds like an "s" or is silent. But because ans starts with a vowel, you must make a liaison. The "x" links to the next word and makes a "z" sound.