Mat 5:21 Ye have heard that it was said by them of old time, Thou shalt not kill; and whosoever shall kill shall be in danger of the judgment:
You have heard what was proclaimed by/to the ancients: "You might not want to murder". That one, however, who possibly murdered is going to bound by the decision.
The two parts of this verse are actually in oppositions to each other, though that is hidden in translation. This is the first "commandment updates" in which Christ fulfills his promise made earlier in Mat 5:17, that he will "complete", "fill up," or, as the KJV translated "fulfill" the law. As we see here and in the following verses, he is not just fulfilling the prophecies but actually completing the law.
The word translated as "have heard," is the most common verb that Christ uses meaning "to hear". It also means "to listen" and "to understand".
The verb translated as "it was said," however, is an uncommon verb for Christ to use to refer to simply saying or speaking, used primarily in the "commandment updates" of this section. There are three other verbs that mean "to say," "to speak," and "to tell," that are much more commonly used. This verb is a more serious meaning, "to proclaim" or "to pronounce," which better fits the context. However, Christ often uses more unusual words for humorous effect, which seems to the case here.
The word translated as "By them of old time" is an adjective used as a noun. It means "beginning" or "source" as an adjective and, as a noun, "the ancients" or "the beginning". It is in the form that usually means "to the ancients," but which can mean "by the ancients" making them an instrument. It could also mean "from the beginning" but Christ uses a different word to express that idea.
The Greek verb translated as "shalt...kill" means "to kill," "to murder," and, more generally, "to stain with blood." The original Hebrew word from Lev 24:17 means "to kill", "to murder," and, more generally, "to strike", "to hit," and, even, "to chastise." The verb indicates something that might happen at some point in time. It is not a commandment. It is in the singular, that is, addressed to one person.
The "not" here is the "not" expressing an opinion, not a fact, that usually has the sense of "you don't want," but it is also the negative used in prohibitions. There is an implicit idea of an opinion regarding making a choice.
The key word here is Greek particle translated as "and" can be a weak form of "and," though it is usually translated as "but" because it joins adversarial statements. More importantly, this how Christ uses this word, to set up a contrast. This means that we have to understand how this second part of the verse is a contrast. The word can also be translated as "however" because it always appears in the second position of the phrase it modifies.
Another Greek word is untranslated in the KJV that means that something "might" happen because it is limited by circumstances. This is clearly in conformance with the Jewish custom where not all forms of striking or even killing were considered wrong, a trial might or might not be warranted.
The Greek verb translated as "shall kill" is the same as above. It is in the third person,
The "shall be" here is the future form of the verb "to be". It is in the future tense, but it a form where the person acts on or for themselves. "Is themselves
The Greek word translated as "in danger", primarily means being "bound" or "tied" but also has the sense of "obligated" to something.
Finally, the Greek word translated as "judgment" has the general meaning of a decision point. However, it also has the meaning of "separating", "choice", "dispute", "event", or "issue". This word is our source of the word "crisis." When referring to the legal proceedings, it means "a trial." However, it is introduced by a definite article, "the decision". It is a form that has a lot of uses in Greek, but the only ones the work well here declare a purpose, "for the decisions" or describe an instrument, "by the decision."
Note, in this verse, Christ does not say that killers are condemned, but that they are liable to a trial. In the Greek, the punishment is even less threatening. The Greek means literally, "bound to trial." This sounds like the modern form due process, rather than what we think of as Biblical justice.
The Spoken Version:
Many snorted while others tittered.
“At some time, you have heard that it was proclaimed by the ancients—.” He then pretended to unroll a scroll. He affected an old man’s wavering voice reading from it, “You might not want to murder. Someone who, however, might possibly murder is going to—.” The speaker paused, looking threateningly at the crowd, then screeched, “Bind himself by the decision!”
Ἠκούσατε (2nd pl aor ind act) "Ye have heard" is from akouo, which means "hear of", "hear tell of", "what one actually hears", "know by hearsay", "listen to", "give ear to", "hear and understand," and "understand."
ὅτι (conj) "That" is from hoti, which introduces a statement of fact "with regard to the fact that", "seeing that," and acts as a causal adverb meaning "for what", "because", "since," and "wherefore."
ἐρρέθη [uncommon](3rd sg aor ind pass) "It hath been said," is from ero, which means "to speak", "to say", "to pronounce", "to tell", "to let suffice", "to announce", "to proclaim," (in passive) "to be pronounced", "to be mentioned", "to be specified", "to be agreed," and "to be promised."
τοῖς ἀρχαίοις (adj pl neut dat) "By them of old time" is from archaios, which means as an adjective means "from the beginning", "from the source", "ancient", "simple", "silly", "former", "the Ancients [of people]", "anciently [adverb], and "the principle [in a loan],"
“Οὐ (partic) "Not" is from ou which is the negative adverb for facts and statements, negating both single words and sentences. The other negative adverb, μή applies to will and thought; οὐ denies, μή rejects; οὐ is absolute, μή relative; οὐ objective, μή subjective.
φονεύσεις:” (2nd sg fut ind act or verb 2nd sg aor subj act) "Thou shalt...kill" is from phoneuō, which means "to kill", "to murder", "to be slain [passive], and "to stain with blood." The original Hebrew word Lev 24:17) was nakah, which meant "to kill", "to murder," and, more generally, "to strike", "to hit," and, even, "to chastise."
ὃς (pron sg masc nom) "Whoever" is from hos, which is the demonstrative pronoun in its various forms (hê, ho, gen. hou, hês, hou, etc. ; dat. pl. hois, hais, hois, etc. gen. hoou). It means "this", "that", "he", "she", "which", "what", "who", "whosoever", "where", "for which reason," and many similar meanings.
δ᾽ (partic) "And" is from de which means "but" and "on the other hand." It is the particle that joins sentences in an adversarial way but can also be a weak connective ("and") and explanation of cause ("so") and a condition ("if").
ἂν (partic) Untranslated is an, which is a particle used with verbs to indicate that the action is limited by circumstances or defined by conditions. There is no exact equivalent in English, but it is translated as "would have", "might", "should," and "could."