He opened his mouth is the literal rendering of a formula that is found in classical literature and in Jewish rabbinical sources. Scholars classify it as both “a Semitic idiom” (Dan 10.16; Job 3.1; 33.2) and “a traditional formula” (Acts 8.35; 10.34). On the basis of its usage in Daniel 10.16 and Job 3.1, at least one commentator believes that it functions to give emphasis to what follows, and that may well be its function here. However, the more acceptable solution is that which understands the expression to mean “begin (verb of) discourse” (see Good News Translation “began to teach”).

Taught them: the verb “teach” may have as its object either the person who is taught or the content of the teaching; in certain contexts it may be followed by both objects. But in Matthew the content of the teaching appears as an object of the verb only in 15.9, which is within a quotation from Isaiah 29.13. Interesting also is the location of Jesus’ teaching in this Gospel; it is always done away from the crowded places, whether on a hill or along the shore of Lake Galilee.

The nearest antecedent to them is “his disciples” (verse 1). It may be necessary in the receptor language to make explicit the persons intended by the pronoun. Some translators interpret them in a wider sense by saying “all the people,” “the people,” or “the crowd.” But it is probably better to leave it as “them” or “his disciples,” as we said above.

The use of the participle saying also reflects Semitic style in which the participle serves merely to indicate that direct discourse is to follow. It then becomes equivalent to quotation marks at the beginning of direct discourse in English. He opened his mouth and taught them, saying is best represented in English as “he began to teach them,” followed by quotation marks. Accordingly, New English Bible restructures as “… he began to address them. And this is the teaching he gave”; Phillips has “Then he began his teaching by saying to them.” Barclay seems to have overtranslated the expression: “He opened his mind and heart to them, and this was the substance of his teaching.” Luke begins his discourse with a different, yet equally challenging structure: “he lifted up his eyes” (6.20).

Quoted with permission from Newman, Barclay M. and Stine, Philip C. A Handbook on the Gospel of Matthew. (UBS Handbook Series). New York: UBS, 1988. For this and other handbooks for translators see here .