Breakdown of Que les lumières sont splendides !
être
to be
la lumière
the light
que
how (exclamative)
splendide
splendid
Questions & Answers about Que les lumières sont splendides !
Why is the adjective splendides at the very end of the sentence?
In French exclamations, the word order stays exactly like a normal statement: subject + verb + adjective. Unlike English, you do not pull the adjective to the front next to the exclamative word.
Could I use Comme or Ce que instead of Que in this sentence?
Could I use Quelles here, as in Quelles les lumières sont splendides !?
No, because Quel (and its feminine plural form Quelles) is an exclamative adjective, meaning it must attach directly to a noun. You could say Quelles lumières splendides ! which means 'What splendid lights!'. To say 'How splendid the lights are!' using a subject and verb, you must use an exclamative adverb like Que, Comme, or Ce que.
Since Que has so many different meanings, how can I quickly tell it means 'How' in this sentence?
The exclamation point is your biggest clue! Grammatically, when a sentence starts with Que, has standard subject-verb word order without inversion, and stands alone as an independent clause, it is typically an exclamation meaning 'How' or 'So much'.