All, therefore, whatsoever - That is, all that they teach that is consistent with the Law of Moses - all the commands of Moses which they read to you and properly explain. The word "all" could not be taken without such a restriction, for Christ himself accuses them of teaching many things contrary to that law, and of making it void by their traditions, Matthew 15:1-6.
They say, and do not - The interpretation which they give to the law is in the main correct, but their lives do not correspond with their teaching. It is not the duty of people to imitate their teachers unless their lives are pure; they are to obey the law of God, and not to frame their lives by the example of evil people.

All therefore whatsoever - That is, all those things which they read out of the law and prophets, and all things which they teach consistently with them. This must be our Lord's meaning: he could not have desired them to do every thing, without restriction, which the Jewish doctors taught; because himself warns his disciples against their false teaching, and testifies that they had made the word of God of none effect by their traditions. See Matthew 15:6, etc. Besides, as our Lord speaks here in the past tense - whatsoever they Have commanded, ὁσα ειπωσιν, he may refer to the teaching of a former period, when they taught the way of God in truth, or were much less corrupted than they were now.

(b) All therefore whatsoever they bid you observe, [that] observe and do; but do not ye after their works: for they say, and do not.
(b) Provided always that they deliver the doctrine of Moses which they profess, which thing the metaphor of the seat shows, which they occupied as teachers of Moses' teaching.

All therefore whatsoever they bid you observe,.... This must be restrained to things that were agreeable to the chair of Moses, in which they sat, to the law of Moses, which they read and explained, to other parts of Scripture and truth in general; for otherwise many of their glosses and traditions were repugnant to the law, and ought not to be observed, as appears from Matthew 5:1. The word "observe", in this clause, is omitted by the Vulgate Latin, Arabic, and Ethiopic versions, and in Munster's Hebrew Gospel; and Beza says, it is wanting in one ancient copy, but is in others; and is retained in the Syriac and Persic versions
that observe and do; hearken to what they say, give diligent heed unto it, take notice of it, and act according to it:
but do not ye after their works; let their doctrine be the rule of your lives, so far as it agrees with the law of Moses; but let not their actions be drawn into an example by you; conform to their instructions, but do not imitate their practices:
for they say, and do not; they talk of good works, but do none; they bid others do them, but do not practise them themselves; they very strictly and severely enjoin them on others, but are very careless themselves to observe them; and of this the Jews are so conscious, that they suggest the same doctrine (n).
"The daughter of Ahar (a wicked man) came before Rabbi; she said to him, Rabbi, supply me with the necessaries of life: he replied to her, daughter, who art thou? she answered him, the daughter of Ahar: he said to her, is there any of his seed in the world? for lo! it is written, Job 18:19. "He shall neither have son, nor nephew, among his people, nor any remaining, in his dwellings": she replied to him, , "remember his law, or doctrine, but do not remember his works."--Says R. Jochanan, what is that which is written, Malachi 2:7. "For the priest's lips should keep knowledge, and they should seek the law at his mouth; for he is the messenger of the Lord of hosts." If the doctor is like to an angel, or messenger of the Lord of hosts, they should seek the law at his mouth; and if not, they should not seek the law at his mouth. Says Resh Lekish, R. Meir found and explained that Scripture, Proverbs 22:17. "Bow down thine ear, and hear the words of the wise, and apply thine heart to my knowledge": to their knowledge it is not said, but to my knowledge. R. Chanina says, hence, Psalm 45:10. "Hearken, O daughter! and consider, incline thine ear, forget thine own people, and thy father's house": on which the gloss is, forget their works, and do not learn them: he that knows how to take care not to learn their works, may learn the law from their mouths.''
--And a little after,
"the disciples of the wise men are like to a nut; as a nut, though it is defiled with mire and filth, yet that which is within it is not to be rejected; so a scholar, or a disciple of a wise man, though he act wickedly, his law, or doctrine, is not to be despised.''
Good doctrine is not the worse for being taught by bad men; nor are good works to be slighted and neglected, because they are not done by all that teach them; but it must be owned that examples are very useful and forcible, and practice greatly recommends doctrine; and it is to be wished, that they both always went together.
(n) T. Bab. Chagiga, fol. 15. 2.

All therefore--that is, all which, as sitting in that seat and teaching out of that law.
they bid you observe, that observe and do--The word "therefore" is thus, it will be seen, of great importance, as limiting those injunctions which He would have them obey to what they fetched from the law itself. In requiring implicit obedience to such injunctions, He would have them to recognize the authority with which they taught over and above the obligations of the law itself--an important principle truly; but He who denounced the traditions of such teachers (Matthew 15:3) cannot have meant here to throw His shield over these. It is remarked by WEBSTER and WILKINSON that the warning to beware of the scribes is given by Mark and Luke (Mark 12:38; Luke 20:46) without any qualification: the charge to respect and obey them being reported by Matthew alone, indicating for whom this Gospel was especially written, and the writer's desire to conciliate the Jews.

Whatsoever they bid you, that observe and do. While in Moses' seat, presenting the law of Moses. He has elsewhere taught that the traditions they added were to be rejected (Matt. chap. 15).
Do not after their works. Do not follow their examples. The law of Moses was still in force, for the Christian dispensation was not ushered in until Christ died, and hence was still to be obeyed, but the wicked example of its teachers was to be rejected.

All things therefore - Which they read out of the law, and enforce therefrom.