Luke 15:11 A certain man had two sons:
A man, a someone, has two sons.
Though the story in Matthew 21:28 seems in translation to begin in the same way, it is completely different. There is no "certain" in that verse, the verb form is different, the word for "sons" is different, and the two is after the noun instead of before it.
The word translated as "a certain" means primarily "anything" or "anyone," but can be used to mean someone of note as we would say "a someone". It appears after the noun, not before it.
The Greek word for "man" means "man", "person" and "humanity" in the singular. In the plural, it means "people" and "peoples". Here it is singular.
The word translated as "have" means to "have", "possess" or "keep". It isn't used to form past tenses like it is in English.
The Greek word for "two" means "two" or a "couple."
The word translated as "sons" more generally means "child." It refers to all offspring in later generations, just like "father" refers to all previous generations. Jesus also used it metaphorically to describe those that follow a way of thought or set of beliefs that descend from an individual. More about it in this article.
Ἄνθρωπός (noun sg masc nom) "Of man" is anthropos, which is "man," and, in plural, "mankind." It also means "humanity" and that which is human and opposed to that which is animal or inanimate. --
τις (pron sg masc nom) "Certain" is tis which can mean "someone", "any one", "everyone", "they [indefinite]", "many a one", "whoever", "anyone", "anything", "some sort", "some sort of", "each", "any", "the individual", "such," and so on. In a question, it can mean "who", "why," or "what."
εῖχεν (verb 3rd sg imperf ind act) "Had" is echo, which means "to have", "to hold", "to possess", "to keep", "to have charge of", "to maintain", "to hold fast", "to bear", "to carry", "to keep close", "to keep safe," and "to have means to do."
δύο (numeral) "Two" is duo, which means the number "two", "a couple," and "a pair."
υἱούς. (noun pl masc acc) "Sons" is huios, which means a "son," and more generally, a "child." It is used generally to refer to any male descendant. -