Gen 32:29 says:
וַיֹּאמֶר, לֹא יַעֲקֹב יֵאָמֵר עוֹד שִׁמְךָ...
Thy name shall be called no more Yaakov...
But in 30 it says:
וַיִּשְׁאַל יַעֲקֹב, וַיֹּאמֶר הַגִּידָה-נָּא שְׁמֶךָ,...
And Yaakov asked him, and said: 'Tell me, I pray thee, thy name.' ...
"Your name" is vocalized differently between the two. Why do we sometimes read shimkha and sometimes shmekha?
Many words have a special form used only when the word occurs at a point in the sentence when a reader would pause while reading, called the 'pausal form'. 'Shmekha' with the segol under the mem is the pausal form of this word; 'Shimkha' is the regular, non-pausal form. Whether a word occurs at a point of pause or flows on to the next word is indicated by the accents, the ta'amei ha-mikra. Every word has an accent and all accents are either conjunctive or disjunctive. There are different intensities of pause indicated by different disjunctive ta'amim. The most intense disjunctive accents, sof pasuk and etnahta, always lead to the pausal form of a word being used. For the less intense pauses, the choice of whether the pausal or non-pausal forms is used seems harder to predict but is determined by the tradition of how to read the Bible as handed down by the Ba'aley Ha-Massorah.
