Mat 15:9 But in vain they do worship me, teaching for doctrines the commandments of men.
Falsely, but they fear me, instructing the [official] instructions, the orders of men.
Lots of interesting stuff here including so evidence that Christ and his followers referred to the old Testament word in the Septuagint, not the Hebrew. This verse with the previous one, quote Isaiah 29:13. The KJV translation of this verse is misleading, with the KJV translation of the old Testament Hebrew coming closer to the meaning of the Greek here. In the original Hebrew, this line is translated in KJV as saying, that "the fear of God comes from the teaching of men, not from God." The Greek could be translated similarly. Christ is actually telling people that they have nothing to fear from God, that the fear of God is taught to enforce the orders of men.
The Greek word translated as "but" joins phrases in an adversarial way. Since it always falls in the second position, translating it as "however" often captures its feeling better.
The word translated as "in vain," is an adverb that also means "fruitless", "at random", "idly," and "falsely."
"Worship" is from a verb that means "feel awe", "full of awe", "feel fear" or "feel shame," before God, "revere, ""honor", "respect," and "worship". Notice this word also means "honor" and "revere" echoing the sense of another Greek word used in Mat 15:6 (And he need not honor his father nor his mother) that started this discussion.. This verb is rarely used by Christ but is used because it is the Greek word used in the Greek Septuagint. Note that its primary meaning is to "feel awe" or "fear," not worship. In original Hebrew, the sense of fear is more direct. The single word, yir'ah, which means "fear", "awe," and specifically, "fear of God."
"Teaching" is a verb that means "to teach", "to instruct", "to explain," and "to give sign of." It is in the form of a participle, so "teaching" or "explaining."
"Doctrines" is the noun form of the verb above, so it which means "teachings", "instruction", "explanations," and "official instruction." There is no "for" in the Greek before this word. It is simply the object of the verb. The verb and this object do not appear together in the Septuagint, so Christ creates an alliteration by putting them together.
The Greek translated as "commandments" means " is a rare form of another Greek noun that means "injunction", "order," and "command." This form is used only here, in the parallel verse, Mar 7:7, and in the Septuagint version. Except for here, quoting the Septuagint, Christ always used the common form.
The sense of this verse and the previous one (Mat 15:8) is to say that people think mentally they love God, but they have no feeling for God or relationship with Him. They falsely feel fear God because they teach the thoughts and preconception of men.
Interestingly, there is a real difference between the English translation of the Greek and the original Hebrew that doesn't exist between the Hebrew and the Greek. The English, at least of the King James, clearly did not look at the Hebrew version when creating a translation.
Christ switches around the wording of the Septuagint to make an alliteration, something like "teaching the teachings" or "indoctrinating the doctrines." This shows a kind of playfulness with scripture.
δὲ "But" 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"). --
σέβονταί ( verb 3rd pl pres ind mp) "Do they worship" is from sebomai, which means "feel awe", "full of awe", "feel fear" or "feel shame," before God, "fear to do", "revere, ""honour", "respect", "approve", and "worship". In Hebrew, the sense of fear is more direct. The single word, yir'ah, which means "fear", "awe," or "fear of God." In the original Hebrew, this line is translated in KJV as saying, that the fear of God comes from the teaching of men, not from God. The Greek could and should be translated this way as well.
ἐντάλματα (noun pl neut acc) "Commandments" is from entalma, which means "to order". The Greek, entalma, only appears here and in the parallel verse, Mar 7:7. It is a form of entolê which means "injunction", "order," and "command."
ἀνθρώπων. (noun pl masc gen) "Of man" is from anthropos, which is "man," and, in plural, "mankind." It also means "humanity" and that which is human and opposed to that which is animal or inanimate. -- The Greek word for "of man" in the singular means "person" and "humanity" and "people" and "peoples" in plural.
Septuagint version is slightly different, reversing putting "commandments" before "doctrines::
μάτην δὲ σέβονταί με διδάσκοντες ἐντάλματα ἀνθρώπων καὶ διδασκαλίας