In Modern Hebrew, a pataḥ makes the same sound as a qamatz, as does the ḥaṭaf pataḥ (Hebrew: חֲטַף פַּתַחIPA:[ħaˈtˤafpaˈtaħ], "reduced pataḥ"). The reduced (or ḥaṭaf) niqqud exist for pataḥ, qamatz, and segol which contain a shva next to it.
In Yiddish orthography, a pataḥ (called pasekh in Yiddish) has two uses. The combination of pasekh with the letter aleph, אַ, is used to represent the vowel [a]; the combination of pasekh with a digraph consisting of two yods, ײַ, is used to represent the diphthong [aj].
A pataḥ on a letter ח, ע, or הּ (that is, ה with a dot (mappiq) in it) at the end of a word is sounded before the letter, and not after. Thus, נֹחַ (Noah; properly transliterated as Noaḥ) is pronounced /no.aχ/ in Modern Hebrew and /no.aħ/ or /no.ʔaħ/ in Biblical Hebrew. This only occurs at the ends of words, only with pataḥ and only with these three letters. This is sometimes called a pataḥ gnuva, or "stolen" pataḥ (more formally, "furtive pataḥ"), since the sound "steals" an imaginary epenthetic consonant to make the extra syllable.
Vowel length comparison
By adding two vertical dots (shva) the vowel is made very short. However, these vowels lengths are not manifested in Modern Hebrew.
