"Your dad is not reading a book."
Translation:אבא שלכם לא קורא ספר.
It is because אבא is Aramaic and in Aramaic the definite article goes after the noun. (Jews learned Aramaic during the Babylonian exile and brought it back with them when Cyrus allowed them back to the Land to rebuild the Temple. That's why Josephus' mother language in first-century Galilee was Aramaic, although he wrote in Greek as the lingua franca of the Roman Empire.) Since Hebrew is a cognate language to Aramaic, they have much vocabulary in common. For instance, "the camel" in Hebrew is הגמל while in Aramaic it is גמלא. The vowels are slightly different, too. The Bavli (Babylonian Talmud) is in Babylonian Jewish Aramaic and the Yerushalmi (Palestinian or Jerusalem Talmud) is in Hebrew and Jewish Palestinian Aramaic. Some vocabulary in modern Hebrew derives from that Aramaic intersection. Of course, אבא in a sense is Hebrew, as it is also found in Mishnaic Hebrew (sometimes called Rabbinic Hebrew), but even then the form (morphology) is derived from Aramaic. Since in Aramaic the article goes before the noun, it would be redundant to have a definite article in this case: האבא would be odd. Having said that, I'm guessing that some modern Hebrew speakers who do not know the Aramaic etymology might slip in the Hebrew definite article. We would have to hear from an expert in modern Hebrew to know whether this mistake occurs at times.
Apparently you can have האבא, people say it that way, but see above about Aramaic אבא. If you use the more traditional אב, then האב is the way to go, but if you use אבא then you can either do a mixed form, האבא, or understand that אבא already has the definite article because in Aramaic the definite article is at the end of the word instead of the beginning.