Did I miss something on word order? Why can't I say "Ili estas ĉiam feliĉaj?"
that is correct, too!
Why is felicxaj plural
Because it must agree with ili.
There is only one happy, so i don't think it makes sense for it to be plural. We have happiness, but happiness is a singular condition. It doesn't make sense to pluralize it. We have happinesses? What am I missing?
In languages like Esperanto, Spanish, French, etc., adjectives must agree with the noun they modify. Since ili (they) is plural, feliĉaj (happy) must be plural to match it. It's just a grammar rule.
Why can't I say "gxojas" instead of "felicxas"?
Cuz they ili are more than one so you use the plural version of happy
Neniu estas cxiam felicxa.