Is -- This helping verb indicates the present tense of the verb. It is used here to form the present, progressive tense, which doesn't exist in Greek but which can smooth the flow of English sentences.

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

lawful: "Lawful" is a verb, which means "it is possible" and "it is allowed." It generally refers to something within someone's power, or, in this case with the negative, something outside of someone's power. This word does not, as it might seem, refer directly to Biblical law.  This is the active verb in the sentence, all the other verbs are infinitives that describe the action.

to -- This "to" is added because the infinitive form of the verb requires a "to" in English.

do good -- The verb translated as "to do good" means "to do good", "to do well", "to act rightly," and "to exert a beneficial influence." It combines the verb meaning "to make" or "to perform" with the common Greek adjective meaning "useful", "worthwhile," and "of high quality." See this article on "good" and this one on "evil."

on  -- (WP) This word "on" comes from the dative case of the following word that requires the addition of a preposition in English. The most common is a "to" for the English indirect object. However, the translator can choose other prepositions: "with,"  "in,"   "of,"  "as," "by," "for," "at," or "on" depending on the context.  This phrase follows "it is lawful" not to do good."

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. 

sabbath  The word translated as "sabbath" is the Greek version of the Hebrew word "shabbat" meaning "rest" or "day of rest".  

days --  There is no word for"days" here but this captures the plural form of "sabbath."

or "Or" is translated from a Greek word that means primarily "or" but serves as "than" in a comparison.

to -- This "to" is added because the infinitive form of the verb requires a "to" in English.

do evil: The verb translated as "to do evil" means "to do ill", "to play the knave," and "to do mischief," and "to injure." It combines the verb meaning "to make" or "to perform" with the common Greek adjective meaning many different forms of "bad," including "ugly", "low born", "craven," and "ill." In the NT, it is often translated as "evil." More about it in this article.

to -- This "to" is added because the infinitive form of the verb requires a "to" in English.

save -- "Save" is the Greek word that means "to keep alive" when applied to people or "to keep safe" when applied to things. Jesus uses it to mean "rescue" in most cases.

life -- (CW) The word translated here as "soul" is psyche, a common word in Greek, familiar in English, meaning "life," "soul," "consciousness," and "a sense of self." It is used for different aspects of "self," the emotional self, the conscious self, the intellectual self. It has a clear sense of the conscious self and is often translated as "life" in the Gospels. Jesus uses it to specifically mean our identity in our worldly life, the role we play on earth, what we might call the "social self," or what we commonly call our "ego." Since "life" is commonly translated from another Greek word, when "self" doesn't work, "person" offers less confusion. See this article for detail about this word. However, it isn't the common word for "life."

or: "Or" is translated from a Greek word that means primarily "or" but serves as "than" in a comparison.

to -- This "to" is added because the infinitive form of the verb requires a "to" in English.

kill?  -- "Kill" is translated from a Greek word that means "slaughter" more than just "kill" because the base word means "slay." The Greek source has the sense of "kill off," that is, "killing" in a more thorough way. When we talk about "slaughtering" someone, we use it to mean destroying their reputation, the strength of their spirit and ideas as well as physically killing them. This is more the sense here. The word "destroy" also works, but a similar word from a different root meaning "kill" works better as "destroy,"