Shortened Vowels

In the qurʾān there are thousands of vowels pro­nounced short, although written as long.
The most common words are انا۠ /ʾana/ (zero above alif = mute unless pause directly after) and أُوْلَٰٓئِكَ /ulaika/ (circle above waw = always mute)
‒ according to the now common Arab Qurʾānic ortho­graphy intro­duced Giza1924.
‒ In IndoPak they have no sign at all ‒ since 1935 some have a circle = a crime against their own system.
   in Turkey qaṣr, in Persia bil-qaṣri is written beneath them
The most common rule is:
when the last sound of a word is a long vowel
and the first sound of the next word ‒ after a silent alif or direct ‒
is an unvowelled consonant (ḥarf sākin), this consonant trans­forms the syllable before to a closed one --> shortens the vowel.
‒ The ḥarf sākin can be a single letter or the first of geminized ones (a pair, double, tašdīd).

And there are some words shortened because of rhyme,
e.g. in Surah Al-Ahzab (33) and Surah Al-Insan (76)

In these suras most verses end in /a/,
the few verses ending in ا get shortened:

[٠٣٣] الأحزاب

٠١٠إِذۡ جَآءُوكُم مِّن فَوۡقِكُمۡ وَمِنۡ أَسۡفَلَ مِنكُمۡ وَإِذۡ زَاغَتِ ٱلۡأَبۡصَٰرُ وَبَلَغَتِ ٱلۡقُلُوبُ ٱلۡحَنَاجِرَ وَتَظُنُّونَ بِٱللَّهِ ٱلظُّنُونَا۠

٠٦٦ يَوۡمَ تُقَلَّبُ وُجُوهُهُمۡ فِی ٱلنَّارِ يَقُولُونَ يَٰلَيۡتَنَآ أَطَعۡنَا ٱللَّهَ وَأَطَعۡنَا ٱلرَّسُولَا۠

٠٦٧وَقَالُوا۟ رَبَّنَآ إِنَّآ أَطَعۡنَا سَادَتَنَا وَكُبَرَآءَنَا فَأَضَلُّونَا ٱلسَّبِيلَا۠

and 76:15 وَيُطَافُ عَلَيۡهِم بِـَٔانِيَةٖ مِّن فِضَّةٖ وَأَكۡوَابٖ كَانَتۡ قَوَارِيرَا۠

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