At the end of Birkas Hamazon, why do we say 'vezaro mevakesh lachem' and not 'lechem'? Thanks!
The standard grammatical rule is that a word whose first vowel is a segol gets converted to a komotz when that word ends a sentence (or at an etnachta, which functions like a semicolon). This is a verse that ends with "lechem", so it becomes "lachem."
Similarly, Vayeitzei begins that Yaakov left from Be'er Shava. (Would have been Sheva, but it's at the end of a clause). "Ato" (patach-komotz) for "you" becomes "oto" (komotz-komotz) throughout the siddur (look especially at shemoneh esrei) where it ends a clause or sentence.
To elaborate on that:
There are, if my computer-script counted properly, 44 “oto”s in Tanach, vs. 560- something “ato”s (so the komotz accounts for about 7% total, or 1 komotz per 13 patachs). Note also that as far as I can tell, if it's “you are adjective”, the stress is on the last syllable. If it's “adjective are you”, it's on the first syllable. All komotz cases (because they end the clause) are mil'el (stress on the first syllable), but the patach form is also occasionally mil'el, e.g. (but not limited to) a zakef katon.
First instance of "you" with a komotz is Genesis 3:10:
Here's Isaiah's prayer, paraphrased in the siddur but maintaining the relevant vowelization:
And a verse from the Zachor Haftarah, containing two forms of "you" according to the usage described here:
Of course, Psalms, Proverbs, and Job employ a different cantillation system, so what defines a partial or full stop there is quite different.