The substitution of أ
The أ can be substituted for following letters:
ا و ي ( Many examples of such substitutions via conversions and many examples found in the Qur'an also. )
ه ( Very few examples of such substitutions and 1 word مَآء with this substitution found in the Qur'an also. )
ع ( Extremely few examples of such substitutions provided by some scholars. )
1. The أ is necessarily substituted for و and ي when they are final ل of a root and pattern in a word after an augmentative ا , whether the initial of the word is pronounced with fatha, kasra or damma. This substitution is done for a ل of a root and pattern in a word, not with ع as final with elided/dropped ل . Conversion is also done with adventitious/separable ة but not with inseparable ة . Conversion is allowably also done with separable ان of dualization (when the word is not originally formed as a dual). Separable ة etc. can be simply explained that the word with it and without it both exist, while inseparable means that only word with it exists and word without it does not exist at all or does not exist in the same sense or meaning shade.
Note: ا أ = آء
Examples:
بَنَّآء with أ substituted for ي , was originally بَنَّاي and has pattern فَعَّال and root ب ن ي .
سَمَآء with أ substituted for و , was originally سَمَاو and has pattern فَعَال and root س م و .
تِلْقَآء with أ substituted for ي , was originally تِلْقَاي , and has pattern تِفْعَال and root ل ق ي .
دُعَآء with أ substituted for و , was originally دُعَاو , and has pattern فُعَال and root د ع و .
شِفَآء with أ substituted for ي , was originally شِفَاي , and has pattern فِعَال and root ش ف ي .
بَنَّآءَة with أ substituted for ي , was originally بَنَّايَة , and has pattern فَعَّالَة and root ب ن ي .
كِسَآءَان with أ substituted optionally for و i.e. original form كِسَاوَان can also be written, was originally كِسَاوَان , and has pattern فِعَالَان and root ك س و .
Advanced Notes on point 1 (Optional reading) :
First, ا is substituted for و and ي via conversion and then changed to أ .The و and ي are mobile after fatha with no barrier except quiescent and augmentative ا which is not an insuperable barrier because of its quiescence and augmentativeness. Moreover, they are in the seat of alteration i.e. end of the word. The 2 quiescents then occurring together, the 2nd of them is converted to أ . The ا and أ are from the same outlet.
2. The أ is necessarily substituted for و and ي when they are ع of a pattern and root in a form I active participle فَاعِل , for which the corresponding perfect form I verb has its و or ي at ع location converted to ا giving فَالَ . This conversion in فَاعِل is still done even if there is ة or plural augments ون / ين / ات etc or dual augments ان / يْن attached to the active participle.
This substitution is also current in patterns فَاعِل and فَاعِلَة etc. even when they are not active participle.
Examples:
قَآئِل with أ substituted for و , was originally قَاوِل , and has pattern فَاعِل and root ق و ل .
غَآئِبِين with أ substituted for ي , was originally غَايِبِين , and has pattern فَاعِلِين and root غ ي ب .
قَآئِمَة with أ substituted for و , was originally قَاوِمَة , and has pattern فَاعِلَة and root ق و م .
مَآئِدَة with أ substituted for ي , was originally مَايِدَة , and has pattern فَاعِلَة and root م ي د .
Advanced Notes on point 2 (Optional reading) :
First, ا is substituted for و and ي via conversion and then changed to أ .The و and ي are mobile after fatha with no barrier except quiescent and augmentative ا which is not an insuperable barrier because of its quiescence and augmentativeness.
3. The أ is necessarily substituted for augmentative ا and و and ي when they occur after ا in plural patterns similar to مَفَاعِل in terms of letters and harkaat e.g. patterns like فَعَاوِل and فَعَايِل which are converted to فَعَآئِل , while in singular they are augmentative letter of prolongation as 3rd letter in the word like فَعُول and فَعِيلَة .
Examples:
شَعَآئِر on pattern فَعَآئِل converted from فَعَايِل which is plural of شَعِيرَة on pattern فَعِيلَة with root ش ع ر .
قَلَآئِد on pattern فَعَآئِل which is plural of قِلَادَة on pattern فِعَالَة with root ق ل د .
عَجَآئِز on pattern فَعَآئِل converted from فَعَاوِل which is plural of عَجُوز on pattern فَعُول with root ع ج ز .
4. The أ is necessarily substituted for و and ي which occur as 2nd of 2 soft letters separated by ا in plural patterns similar to مَفَاعِل in terms of letters and harkaat. With ع of root and pattern also a و or ي , the possible substitutions can be explained like below :
فَوَآئِل which can be originally فَوَاوِل or فَوَايِل .
فَيَآئِل which can be originally فَيَاوِل or فَيَايِل .
5. When 2 و come together at the beginning of a word i.e. as 1st and 2nd letter of a word.
1st of them is of course mobile i.e. has a vowel and is not quiescent.
If 2nd is also mobile OR
is quiescent or letter of prolongation but :
original as a و /
a و inseparable from the formation /
a و that is not an augmentative letter of prolongation converted from another letter i.e. is not an unoriginal letter of prolongation
Then the 1st و is necessarily converted to أ .
In most other cases of 2 و together, it is allowed to convert 1st و to أ .
Example:
أَوَاصِل with أ substituted for و , was originally وَوَاصِل and has pattern فَوَاعِل and root و ص ل . It is plural of وَاصِل on pattern فَاعِل .
6. Some allowable conversions of و to أ are explained via examples below:
أَثْؤُب with أ substituted for و , was originally أَثْوُب and has pattern أَفْعُل and root ث و ب . It is plural of ثَوْب on pattern فَعْل .
سُؤُوق with أ substituted for و , was originally سُوُوق and has pattern فُعُول and root س و ق. It is plural of سَاق .
Note: ؤ is written in word instead of أ due to damma on it and it being 1 of the middle letters in a word. Similarly قَائِل is written instead of قَاإِل (can't write it like this in Arabic) due to kasra on it and it being 1 of the middle letters in a word.
7. أَحَد and إِحْدَى used in The Qur'an with substitution of أ for و . See below.
أَحَد with أ substituted for و , was originally وَحَد , and has pattern فَعَل and root و ح د .
إِحْدَى with أ substituted for و , was originally وِحْدَى , and has pattern فِعْلَى and root و ح د .
The remaining words from the same root و ح د used in The Qur'an are used without substitution and these include:
وَحْد on pattern فَعْل .
وَحِيد on pattern فَعِيل .
وَاحِد on pattern فَاعِل .
وَاحِدَة on pattern فَاعِلَة .
8. مَآء used in the Qur'an with substitution of أ for ه and substitution via conversion of ا for و was originally مَوَه and has pattern فَعَل and root م و ه .
.