- Disambiguation: see salary
Compare buy - when you buy something you have to pay for it.
Pay is an irregular lexical verb, although only the spelling is irregular. The third person form is pays, the -ing form "paying", the preterite and past participle are "paid". In this regard the spelling is identical to say, though not the pronunciation.
Pay has an obvious French true friend, "payer" (infinitive).
A second, far less common, usage of pay means "to seal the decks of a ship with tar". This has a different etymological origin. This verb is a regular verb, with the preterite and past participle "payed" - though often "paid" is used here as well. This article is not about this sense of pay, which is now obscure, increasingly obsolete and probably worth ignoring.