It  -- This is from the third-person, singular form of the verb.

is -- (WT) This helping verb indicates passive form and the present tense of the verb. Though "it is written," is translated in the present tense in the KJV, the Greek is in a tense that indicates an action completed in the past.

written, -- "Written" is the Greek verb that  means "to mark," "to express by written characters," "to write a letter," "to write down [a law]," and so on.

That  - The word translated as "that" introduces a statement of fact or cause. Here it introduces a quote. 

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. 

man -- The word translated as "man" more generally means "humanity" as well as "a man."

shall -- This helping verb "shall" indicates that the verb is the future tense. Helping or auxiliary verbs are needed to translate the Greek verb forms into English.

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.

live -- The verb means "live," "to be alive,” “to be full of life," "to be strong," and "to be fresh." So it is life in the sense of a vital life, strong and growing. Perhaps in English, “thrive” would be more precise. It is in the future tense, which we cannot tell by the “shall live” because the KJV and other Bibles translated both the future tense and the subjective mood in the same way, so “he going to thrive.” But again, the form is the middle voice, so “by himself.” So this verb, with the negatives means “he is not going to thrive by himself.” This is a pretty interesting statement even without the “bread” part. Christ also uses it in the sense of "making a living." There are several other words in the Gospels translated as "life" discussed in this article. The differences are important.- The term translated as "live" is in the future tense. The Greek term means not only to have  life and breath but to the enjoyment of living as well, specifically, to be "full of life."

by -- The word translated as "by" means "on," "over," "upon," "against," "before," "after," "during," "by" or "on." The primary meaning of this word is "upon," which works well for the idea of "living on bread."

bread -- The Greek word for "bread" or more precisely, "loaf," but is was used generically to mean "food." The original Hebrew (Deu 8:3) from which this quote is taken referred to mana in the desert. Moses describes the hunger and suffering of the Jews in the desert and how God sent them mana. The following verse of Deu 8:5 may also be considered a provocative prophecy of Christ's suffering and death.

alone, -- "Alone" is an adjective modifying "bread" that means "alone," "solitary," "only," "single," "unique," "made in one piece," "without [someone]," "only [something]," "unique," "one above all others," and "on one condition only."

, but by every word of God. -- (OS) There is nothing in the Greek that can be translated as "but by every word of God" in the source we use today but it does exist in the source that the KJV translators used.