The MT for Isaiah 9:5 is:
כִּי־יֶ֣לֶד יֻלַּד־לָ֗נוּ בֵּ֚ן נִתַּן־לָ֔נוּ וַתְּהִ֥י הַמִּשְׂרָ֖ה עַל־שִׁכְמ֑וֹ וַיִּקְרָ֨א שְׁמ֜וֹ פֶּ֠לֶא יוֹעֵץ֙ אֵ֣ל גִּבּ֔וֹר אֲבִי־עַ֖ד שַׂר־שָׁלֽוֹם
The verse ends with a list of nouns or noun phrases:
- פֶּ֠לֶא - wonder
- יוֹעֵץ֙ - counselor
- אֵ֣ל גִּבּ֔וֹר - God is great
- אֲבִי־עַ֖ד - eternal father
- שַׂר־שָׁלֽוֹם - minister of peace
There is nothing in the MT, neither in the consonantal text nor in the Masoretic tradition (the diacritics and readers marks) that would indicate that the verse should not be read as if with commas after each noun:
For to us a child is born, to us a son is given; and the government shall be upon his shoulders, and his name shall be called Wonder, Counselor, Mighty God, Everlasting Father, Prince of Peace.
And פלא should be translated as a noun, "Wonder", not as an adjective, and followed by a comma.
The readers marks imply the commas, however, the intent of the consonantal text is apparently that the list is actually one long name, with no commas at all, as if it were:
For to us a child is born, to us a son is given; and the government shall be upon his shoulders, and his name shall be called Wonder-Counselor-Mighty-God-Everlasting-Father-Prince-of-Peace.
since "his name" is singular.
The commentators and translators who translate "Wonderful Counselor," rather then "Wonder, Counselor," see פלא יועץ as a construct state noun phrase. This has aesthetic appeal in that the name list becomes a list of four noun construct state phrases. However, this interpretation reads heavily against the readers mark for "wonder", a tilsha yemina, which is a hard stop type of mark, indicating a comma or period. There is also no hyphenation in in the MT between "wonder" and "counselor" as there is in the other noun constructs. In addition, the phrase "פלא יועץ" presents problems in Hebrew. The meaning would seem to be "a counselor of wonders", which does not make sense with the way פלא, wonder, is used in the MT or the way יועץ, leader or counselor is used. If the intent were really "wonderful counselor" we would expect the order to be reversed, "יועץ פלא". Nonetheless, in later Hebrew, the phrase "פלא יועץ" took on a life of its own, and came to mean "wonderful counsel" (not "wonderful counselor"). From there it was probably back read into the translations.