For  - (OS) There is no Greek word in best Greek sources that can be translated as "for," although such a word appeared in the Greek source used by the KJV translators.

this  - -- "This" is translated from a Greek word that means "this," "that," "the nearer."

is -- The verb "is" here is the common form of "to be" in Greek. It means to have a certain characteristic or remain in a certain condition. It also equates terms or assigns characteristics. 

he, -- (IW) There is nothing that can be translated as "he" in the Greek source.

of -- The Greek word translated as "of" means It means "around" when referring to a place, but, in this context, it means "about," "concerning," "on account of," and "in regard to." This is the way Christ usually uses it.

whom -- The word translated as "whom" is a demonstrative pronoun ("this" "that"), but it often acts as a pronoun ("the one that), especially a connective pronoun ("the one that") introducing a dependent clause.

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

is -- (WT) This helping verb "is" indicates that the following verb is passive. Helping or auxiliary verbs are needed to translate the Greek verb forms into English. However, the tense here is the past perfect, so "has been."

written -- "Written" is a verb that means "to mark," "to express by written characters," and "to write down [a law]."  The verb is passive, completed in the past, "it has been written."

Behold, "Behold" is an adverb meaning "Lo! Behold!" and "See there!" This word did not appear in the original Hebrew. It was added as a rhetorical flourish by the writers of the Septuagint. 

I  - The pronoun "I" is used here. Since, as the subject of the sentence, it is part of the verb, its explicit use accentuates who is speaking "I." Saying "I myself" captures this feeling in English. 

missing "myself" -- (MW)  The subjective pronoun repeats the information in the verb so it should be repeated in English like "I myself."

send   - The "send " here is a word that means "to send off" and "dispatch." It is the source of our word "apostle." The word is a little different in the Septuagint, adding a prefix meaning "out of" or "from" to the beginning of this word. -

my -- "My" is the first-person possessive singular pronoun. This pronoun follows the noun so "of mine."

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. 

messenger  - "Messenger" is a word that is the source of our word, angel. It means "messenger" and "envoy." This is one of the rare occasions when it is not translated as "angels" in the NT. The Hebrew word in the original OT verse is mal'ak, which is also usually translated as "angel" in the English, However, it was always translated as "messenger" in the Greek Septuagint. "Messenger" is a word that is the source of our word, angel. It means "messenger" and "envoy." This is one of the rare occasions when it is not translated as "angels" in the NT. The Hebrew word in the original OT verse is mal'ak, which is also usually translated as "angel" in the English, However, it was always translated as "messenger" in the Greek Septuagint. 

before  - The word translated as "before" has many meanings of before, including before in time. This word is not used in the Septuagint version. 

thy   -- The word translated as "thy" is the genitive form of the singular, second-person pronoun, which is most commonly the possessive form. This pronoun follows the noun so "of yours." In the Septuagint, this was "of mine" with God referring to is own face. At this point, Christ changes the verse's pronouns from those in the Septuagint: from the first person "my," that is, God referring to himself, to "thy" or "your," God referring to Christ. 

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. 

face,  - "Face" is a Greek word that means "face," "countenance," "front," and "bodily presence." This word is used later in the Septuagint version.

which  - -- The word translated as "which" is a demonstrative pronoun ("this" "that"), but it often acts as a pronoun ("the one that), especially a connective pronoun ("the one that") introducing a dependent clause.

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.

prepare  - (CW) "Prepare" is a word that means "to equip," "to prove," "to construct," and "to prepare oneself." It is in the future tense. Christ uses this word only in this quote here and in Luke 7:26, the parallel quote in Luke. A completely different Greek word is used in the Septuagint. This word is more common in ancient Greek than the one in the Septuagint.

thy -- The word translated as "thy" is the genitive form of the singular, second-person pronoun, which is most commonly the possessive form. This pronoun follows the noun so "of yours." There is no possessive pronoun in the Septuagint version.

way - "Way" is a word that means literally, "a threshold," but has all of the same meaning of "way" in English. It means "a road," "further along the road," "a journey," "a manner of doing things," and "a system." In the Septuagint, there is no "your' here, it is "a way." 

before  - A different Greek word for "before" is used here that the one used earlier in the verse. The point may be to create a contrast between the ideas of "before" in time and "before" in place. This is the preposition used in the Septuagint.

thee -- The "you" is the second person pronoun. In the Septuagint version the phrase "my face" appears here.  In the Septuagint, this is "me."