Apostles asked why they were unable to caste out a demon.
Matthew 17:21 Howbeit this kind goeth not out but by prayer and fasting.
This one, however, this kind, doesn't go out except by prayer. And it is fasting.
People like to put words in Jesus's mouth.
This verse does not appear in the sources we used today. Below is the verse from the source used by the KJV translators.
Textus Receptus: τοῦτο δὲ τὸ γένος οὐκ ἐκπορεύεται εἰ μὴ ἐν προσευχῇ καὶ νηστείᾳ
This verse does not exist in the sources we use today, and highly suspicious because of its content. The analysis is of the Textus Receptus version used by the KJV translators. The version in Mark 9:29 is shorter and uses a slightly different vocabulary. Many of these words are uncommon for Jesus and the final word, "fasting," is a feminine form only used here. It seems to be the wrong form, a subject rather than the object of the preposition.
TEXTUS RECEPTUS ANALYSIS
Howbeit -- (CW) The Greek word translated as "howbeit" means "but," "however," and "on the other hand." Since it always falls in the second position, translating it as "however" often captures its feeling better.
this -- The word translated as "this" means "from here" or "this/that thing."
missing "the" -- (MW) The untranslated word is the Greek definite article, which usually precedes a noun and, without a noun, takes the meaning of "the one." The Greek article is much closer to our demonstrative pronouns ("this," "that," "these," "those"). See this article for more.
kind -- (CW)"Kind" is from an uncommon noun for Jesus that means "race," "offspring," "class," "sort," and "kind." The female form of the noun is the word translated as "generation."
goeth..out -- "Goes out" is from a verb which means "to make to go out" and "fetch out."
not -- The Greek word translated as "not" is the Greek negative used to deny objective facts, not opinions. It makes a negative statement of fact. Adding "really" to the sentence captures the same idea.
but -- (WW) This is not one of the two Greek words usually translated as "but." It is two Greek words are that mean "if not" but this phrase is used to mean "except," "instead," and "but."
by -- The word translated as "by" means "in," "by," "within," "with," "during" (time), or "among" with a dative object as the one here.
prayer -- "Prayer" is from the noun which means only "prayer." The verb from the same root is usually used by Jesus in his teachings. This is the first time that the noun is used.
and -- The Greek word translated as "and" is used as the conjunction "and," but it also is used to add emphasis ("also").
fasting. -- (WF) "Fasting" is from a noun that means only "to fast." However, Jesus usually used the masculine, not feminine, noun form. This form only appears here and it is not a standard dative, looking more like the subject.
Τοῦτο (pron sg neut nom) "That" is touto, which means "from here," "from there," "this [thing]," or "that [thing]."
δὲ (conj/adv) "Howbeit" is de which means "but" and "on the other hand." It is the particle that joins sentences in an adversarial way but can also be an explanation of cause ("so").
τὸ (article sg neut nom) "The" is the Greek definite article, hos, ("the"), which usually precedes a noun and, without a noun, takes the meaning of "the one" or, in the plural, "the ones."
γένος [3 verses]( noun sg neut nom) "Kind" is genos, which means "race," "offspring," "class," "sort," and "kind."
οὐκ (partic) "Not" is ou which is the negative adverb for facts and statements, negating both single words and sentences. The other negative adverb, μή applies to will and thought; οὐ denies, μή rejects; οὐ is absolute, μή relative; οὐ objective, μή subjective.
ἐκπορεύεται [11 verses] (verb sg mp 3rd pres ind) "Goes out" ekporeuômai, which means "to make to go out" and "fetch out."
εἰ μὴ (conj particle) "But" is ei me, which is the conjunction that means "if not," "but," and "except." εἰ is the particle use with the imperative usually to express conditions "if" or indirect questions, "whether." mê (me) is the negative used in prohibitions and expressions of doubt meaning "not" and "no."
ἐν (prep) "In" is en, which means "in," "on," "at," "by," "among," "within," "surrounded by," "in one's hands," "in one's power," and "with."
προσευχῇ [5 verses](noun sg fem nom) "Prayer" is from proseuche, which means only "prayer." The verb form, proseuchomai, is usually used by Jesus in his teachings. This is the first time that the noun is used.
καὶ (conj/adv) "And" is kai, which is the conjunction joining phrases and clauses, "and," or "also." After words implying sameness, "as" (the same opinion as you). Used in series, joins positive with negative "Not only...but also." Also used to give emphasis, "even," "also," and "just."
νηστείᾳ [1 verses]( noun sg fem dat ) "Fasting" is nesteia, which means "to fast."