Learn Swedish: Titta, hela papperslyktan brinner! - Look, the whole paper lantern is burning!

This is the special exception for hela (the whole) and halva (half the). Normally, when an adjective modifies a definite noun, you need the freestanding definite article (like den in den vackra papperslyktan). However, hela and halva attach directly to the definite noun without den, det, or de.

Does hela change if the noun is an ett-word instead of an en-word like lykta?

No, when used to mean "the whole" followed by a definite noun, it is always hela. It doesn't matter if the noun is an en-word or an ett-word. For example, you would say hela huset (the whole house), even though hus is an ett-word.

Why is there an 's' in the middle of papperslyktan?

This is a linking 's' joining the two root nouns papper (paper) and lykta (lantern). It acts as a glue when creating many Swedish compound nouns, somewhat similar to the 's' in the English word "craftsman".

Why use brinner here instead of bränner?