Mat 6:23 But if thine eye be evil, thy whole body shall be full of darkness. If therefore the light that is in thee be darkness, how great is that darkness!
If, however, your eye is worthless, the whole body of yours is going to be dark. If the light, the one in you, is a darkness, the darkness, how much?
This first part of this verse is the counterpoint to Mat 6:22, and completes the previous verse's double meanings and adds a few more for good measure. Again, this verse makes more sense if you understand Christ's use of words meaning "light" and "darkness" (refer to this article). This verse also demonstrates how misleading the English translation of "evil" is (refer to this article on "evil"). Again, everything here is address to a single person.
The Greek word translated as "but" is commonly translated as "but," since it always falls in the second position, translating it as "however" often captures its feeling better.
The Greek word meaning "if " indicates more of an expectation of something happening than "if" alone.
The Greek word for "eye" is the more technical terms for "eye" but it also means "sight". It is a metaphor for "cheer."
The first "be" here is in in a form indicating something that "might be." In English, this form is assumed with the "if" that begins the phrase.
The word translated as "evil" means "second-rate" or "worthless." This makes much more sense in a contrast with a "focused" eye in the previous verse. This article explores the meaning of this Greek in more detail, using this verse as an example of why "evil" doesn't work. Interestingly enough, there is another Greek word combining the Greek words used for "evil" and "eye" that has the meaning of "envious" eye. This adds a whole new layer of meaning what Christ means by "second-rate sight," seeing yourself as second-rate and therefore being jealous of others. This if very different than any concept in Christ's era or ours of the "evil eye".
The word translated as "whole" means "complete" and "entire," but it also has a number of other meanings including "the universe" and "speaking generally."
The words translated as "body" means "the body" but it also means "the substance of things" generally, as we use the term "a body of evidence." Generally, it refers to material existence. Christ uses another Greek word to refer more specifically to his physical body or "flesh". The "your body" gives use the sense of an individual's physical life.
The Greek word translated as "shall be" is the future tense of the common form of "to be."
There are not Greek words here that can be translated as "full of" except as an explanation of the following word and to conform to the previous verse.
The word translated as "full of darkness" means "dark", "blind", "dull", and "in privacy". It is the adjective form of the word that Christ commonly uses to mean "dark (again, more about all these words and their use here). It has no sense of "full of", which is added to conform with the previous verse. Since Christ uses light as a metaphor for knowing, the translation as "dull" works well because in English it means not bright. However, It is also the opposite of "well-known," since it means "obscure." As the opposite of seeing things clearly and distinctly, it means "blind" but Christ typically uses another Greek word to mean "blind".
The second "if" here is a little different Greek word than the "if" that starts this verse, indicating more certainty. This is also reflected in the difference in the verb as well.
The Greek word translated either as "therefore" either emphasizes the truth of something ("certainly", "really") or it simply continues an existing narrative ("therefore", "then"). Because of the word used above, "certainly" seems more certain.
The Greek word translated as "light" is a different, more general word that the Greek word translated as "light" in the previous verse. It has a number of meanings (again, refer to this article). It is introduced with the article so "the light."
The word translated as "that is" is from the Greek article, "the," which usually proceeds a noun and, without a noun, takes the meaning of "the one."
The Greek words translated as "in thee" are the common preposition and the singular "you" pronoun.
The "be" is the normal Greek verb "to be" usually translated as "is" or "it is" in this form.
The Greek word translated as "darkness" has the general meaning of "darkness", "gloom", "blindness" and have many of the opposite meanings of light. Though it has an article making it "the darkness," it doesn't have an article "the" associated with it so "a darkness" is also the sense.
The Greek word translated as "how great" means "how much" and "how many." It is used to question any type of measure. It is commonly used with questions. This word appears at the end of the sentence so it acts as more as a punch line.
There is no "is" associated with this section of the verse, but it is understood. This is a common difference between written and spoken language (see this article).
The Greek word translated as "darkness" is the same as the one above. Though it has an article making it "the darkness," it doesn't have a demonstrative pronoun, "that" associated with it but the Greek article often emphasize more that our "the", acting more like our demonstrative adjectives.
There are many plays here on light and darkness, vision and blindness, happiness and sadness.
The Spoken Version:
“If, however, your eye is worthless,” the speaker continued. “That whole body of yours, it is going to be—dark!”
As he said “dark” or right before, clouds again passed over the sun.
People chuckled nervously at the coincidence. Murmuring spread.
“If the light—the one in you? ” The speaker continued, his voice growing more ominous. “A darkness? It is the darkness so dark!”
ἐὰν (conj) "If" is from ean, which is a conditional particle (derived from ei (if)and an (might)) which makes reference to a time and experience in the future that introduces but does not determine an event.
δὲ (partic) "But" is from de which means "but" and "on the other hand." It is the particle that joins sentences in an adversarial way but can also be a weak connective ("and") and explanation of cause ("so") and a condition ("if").
ὅλον (adj sg neut nom) "Whole" is from holos (holos), which means "the whole", "entire", "complete", "complete in all its parts", "wholly", "altogether", "on the whole", "speaking generally", "utter," "actually", "really, "the universe," and "safe and sound."
εἰ (conj) "If" is from ei, which is the particle used to express conditions "if" (implying nothing about its fulfillment) or indirect questions, "whether." It also means "if ever", "in case," and "whenever." It is combined with various conjunctions to create derivative conditions.
τὸ φῶς (noun sg neut nom) "The light" is from phos, which means "light", "daylight [primarily], "illumination [of things and of the mind]", "light [of the eyes], "window", "opening", "public visibility," and "publicity." Christ uses it as a metaphor for "knowledge," but in Greek it is also a metaphor for "deliverance", "happiness", "victory," and "glory."
τὸ (article sg neut nom) "That" is from is the accusative, masculine, singular article, "the."
πόσον. (adj sg neut nom ) "How great" is from posos, which means "of what quantity," [in distance] "how far." [of number] how far," [of time] "how long," [of value] "how much", "how great", "how many," and "how much."