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."

am -- The verb "am" 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. With the genitive object, the sense is "belongs to."

one -- (CW) The word translated as "one" is the Greek definite article, without a noun, it has the sense of "the one." The Greek article is much closer to our demonstrative pronouns ("this," "that," "these," "those") than the English "the." See this article for more.  This is not the common word for the number "one."

who -- (IW) There is nothing that can be translated as "who " in the Greek source. It was added because the next verb was translated as active rather than as a participle.

testifies-- (WF) "Testifies" is the Greek verb that means "to give testimony" and "to bear witness." It has the sense of being true testimony. It is the verb form of the Greek word for "testimony" and "proof."   The form of the word is not an active verb but a participle.

for -- (WW) The Greek word translated as "of" means "around" when referring to a place, but in referring to a subject, it means "about," "concerning," "on account of," and "in regard to." This is the way Jesus usually uses it. It does not mean "for."

myself, -- The Greek reflexive pronoun is translated as "myself."

missing "about"  -- (MW) The untranslated word  "about" means "around" when referring to a place, but in referring to a subject, it means "about," "concerning," "on account of," and "in regard to." This is the way Jesus usually uses it. It is not the common "of."

my. -- (WF) "My" is the regular first-person adjective in Greek as the object of an untranslated preposition.

other -- (WW) The Greek word translated as "other" is used as the conjunction "and," but it also is used to add emphasis ("also"). In a series, it is can be translated as "not only...but also." After words implying sameness "as."

witness --  (WF) "Witness" is the Greek verb that means "to give testimony" and "to bear witness." It has the sense of being true testimony. It is the verb form of the Greek word for "testimony" and "proof." This is not the noun form but an active verb.

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

the   -- The word translated as "the" is the Greek definite article, without a noun, it has the sense of "the one." The Greek article is much closer to our demonstrative pronouns ("this," "that," "these," "those") than the English "the." See this article for more. 

Father -- "Father" is the Greek noun that means "father" or any male ancestor so "forefathers." It is the word that Christ uses to address his own Father.

who -- (IW) There is nothing that can be translated as "who" in the Greek source. It was added because the next verb was translated as active rather than as a participle.

sent -- (WF) "Sent"is from a Greek verb that means "send," "send forth," "send away," "conduct," and "escort." This is the second most common word Jesus uses that is translated as "send out," but this one doesn't have the prefix that has the sense of "out."  The form of the word is not an active verb but a participle.

me -- "Me" is the regular first-person pronoun in Greek as the object of the verb or preposition.