missing "but"  -- (MW) The untranslated word  "but" means "but," "however," and "on the other hand." It joins phrases in an adversarial way. Since it always falls in the second position, translating it as "however" often captures its feeling better.  It can also be an explanation of cause ("so").

when -- (CW) The Greek word meaning "when" indicates more of an expectation of something happening than "if" alone. This is how we use the word "when." This is not the simple "if.

a -- (IW)There is no indefinite article in Greek, but when a noun doesn't have a definite article, the indefinite article can be added in English translation but the following word is not a noun.

person -- (WW) -- The Greek word translated as "man" in the singular means "anyone," "someone,"  "something," and "anything." As a subject, the word can be used either as masculine or feminine so "anyone" works best for a person. In the plural, it means "some," "they," and "those." Jesus often uses it to start a question so it means "who," "what," or even "why."

walks  -- "Walk" is a Greek verb that means "to walk up and down," "to walk about," and "to walk about while teaching." Jesus uses it somewhat humorously in the sense that we use "parade." The form is one of possibility, but that is implied by the "if/when."

at-- The word translated as "in" means "in," "within," "with" (an instrument), "during" (time),  or "among"  with a dative object as the one here.

missing "the"  -- (MW) The untranslated word "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. 

night, -- "Night"  is the noun that means "night," "midnight," and is a metaphor for darkness. --

they -- (WN) This is from the third-person, singular form of the verb.

stumble -- (CW) This word is only used three times by Jesus. Twice in parallel verses to means "stumble upon," even though it is not the common word for "stumble." Once to describe a storm beating against a house in last story in the  Sermon on the Mount. " The word means "strike against" so in the context of walking, it means "stumble upon."

for - The word translated as "because" introduces a statement of fact or cause, "for what," "because," "since," and "wherefore."

they -- (WN) This is from the third-person, singular form of the verb.

have -- (WW) The verb "have" 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." The word also means "to exist" and where it doesn't connect to characteristics or conditions. -- When the verb "to be" appears early in the clause before the subject, the sense is more like "it is" or, in the plural, "there are."

no -- (WP) The Greek word translated as "not" is the Greek negative used to deny objective facts, not opinions. It means "no," "not," or"no truly." It makes a negative statement of fact. Adding "really" to the sentence captures the same idea. When a negative precedes the verb, it affects the whole clause. This word precedes the verb not the noun, "light."

missing "the"  -- (MW) The untranslated word is the Greek definite article,"the," 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. 

light -- The Greek word translated as "the light" means "light," "daylight [primarily], "opening," and "public visibility." Jesus uses it as a metaphor for "knowledge,"

missing "in"  -- (MW) The untranslated word "in" means "in," "within," "with" (an instrument), "during" (time),  or "among"  with a dative object as the one here. 

missing "him"  -- (MW) The untranslated word "him" is the Greek word commonly translated as third-person pronouns in English.