Learn Swedish: Utställningen pågår under hela sommaren. - The exhibition goes on during the whole summer.

In time expressions, under means "during". It is used for an ongoing period of time to say that an event happens throughout that duration. Here, it shows the exhibition is open throughout the entirety of the summer.

What does pågår mean, and is it the same as saying går på?

pågår translates to "is going on", "is in progress", or "lasts". It is a single word used for events or processes. går på, written as two words, usually means "goes to" or "attends" (like går på bio - goes to the cinema) or physically "walks on".

Why is it hela sommaren with a definite ending on the word for summer?

When you use hela to mean "the whole", the noun that follows must be in the definite form. Just like in English you say "the whole summer" instead of "the whole a summer", Swedish pairs hela with the definite sommaren.

How is the word utställningen formed?

It comes from the phrasal verb att ställa ut, which means "to exhibit" or "to display". The parts are combined and given the -ning suffix to make it a noun, utställning (exhibition). The final -en makes it definite, so it means "the exhibition".