Occasionally, Mounce's explanations don't seem quite right. One example occurs on p. 32 of the 3d edition, in section 6.12 on case endings. As a bit of background, Mounce takes the unusual approach of giving the first and second endings apart from the stem-vowel (alpha/eta or omicron, respectively), rather than, as more common, expect students to learn the endings with the stem vowel.
Mounce's approach, however, runs into a wrinkle for the second declension, nom./acc. neuter plural ending, as in ἔργα (from singular ἔργον), because the omicron stem vowel is no longer there. Here's how Mounce attempts to explain it:
6.12 ... The underline (α means that the case ending joins with the final stem vowel.5 ...
5 This is called "contraction," and I will discuss it in detail later. For example, the stem of the noun ἔργον is ἔργο. When it is in the neuter plural its form is ἔργα. The omicron and alpha have "contracted" to alpha. ἔργο + α > ἔργα.
This explanation is simply wrong. In particular, omicron and alpha do not "contract" to a (short!) alpha. Rather, omicron + alpha contracts to a (long) omega (as Mounce later recognizes in the chart on p. 343).
The explanation is also wrong as a historical matter. The alpha of the neuter plural is not the result of an o-vowel plus an alpha ending; rather, it is the ordinary reflex of the Proto-Indo-European ending of *-eH2, which the vowel *e plus the a-coloring laryngeal H2. In this case, the stem vowel *o changes to *e by ablaut, which then is colored to α by the following H2.
Now, I don't expect introductory grammars of Biblical Greek to be giving the details of Proto-Indo-European morphology, but I do expect that their explanations of the Greek be correct. In this case, a more accurate explanation is that the neuter plural nom./acc. ending -α simply replaces the second declension stem vowel. There is no need to appeal to "contraction," which never occurred historically here and which apparently behaves differently from how omicron and alpha normally contract.