The substitution of ا
The ا can be substituted for following :
أ و ي
1. The ا is regularly substituted via conversion for و and ي when they are mobile (not quiescent) and come after a letter pronounced with fatha in the same word. This conversion of و and ي into ا is subject to following conditions:
i. The و and ي must be mobile e.g. وَ وِ وُ يَ يِ يُ and not quiescent e.g. وْ يْ .
Examples:
قَامَ with ا substituted for و was originally قَوَمَ and has pattern فَعَلَ and root ق و م.
قَوْم with pattern فَعْل and root ق و م , has no substitution of ا for و since و is quiescent وْ .
ii. The letter before و or ي must be pronounced with fatha. So there is no substitution of ا for و or ي if that letter before them is pronounced with kasra or damma.
Examples:
There is no substitution in عِوَج , سُوَر etc.
iii. The و or ي must be preceded by fatha in the same word, if a fatha in another word comes before them, there is no substitution of ا for و or ي .
iv. The vowel of و or ي must be original (& not assigned due to elision (dropping/removing) of another letter in the word).
Examples:
جَابَ with ا substituted for ي was originally جَيَبَ and has pattern فَعَلَ and root ج ي ب .
تَوَم which was formed from original تَوْأَم with pattern فَوْعَل and root ت أ م by dropping أ has no ا substituted for و because the vowel of و is not original. Originally و was quiescent وْ i.e. had sukoon.
v. The conjunction i.e. coming of fatha directly before و or ي must be original, and not adventitious e.g. caused by elision of another letter. There must be no letter between fatha and و or ي in original form of the word also. In other words, if the fatha comes directly before و or ي in a word, but in its original form (before any elision or other transformation) , a letter comes between و or ي , then there is no substitution of ا for و or ي in this word also.
vi. The و or ي when ع of a root in a word must be followed by a mobile letter. If not followed by a mobile letter, then there is no substitution of ا for و or ي .
Examples:
خَافَ with ا substituted for و was originally خَوِفَ and has pattern فَعِلَ and root خ و ف .
There is no substitution in:
بَيَان with root ب ي ن & pattern فَعَال because ي being ع of the root in the word is not followed by a mobile letter.
طَوِيل with root ط و ل & pattern فَعِيل because و being ع of the root in the word is not followed by a mobile letter.
vii. The و or ي when ل of a root in a word must not be immediately followed by an ا or a or a doubled ي i.e. يّ . If followed by any of these 2, then there is no substitution of ا for و or ي .
Examples:
There is no conversion of و to ا in عَلَوِيّ or فَتَوِيّ because و is immediately followed by يّ .
There is no conversion of و to ا in عَصَوَان because و is immediately followed by an ا .
There is no conversion of ي to ا in فَتَيَان because ي is immediately followed by an ا .
بَدَا with ا substituted for و was originally بَدَوَ and has pattern فَعَلَ and root ب د و .
بَغَى with ا substituted for ي was originally بَغَيَ and has pattern فَعَلَ and root ب غ ي .
The ا at the end of this example is written ى but spoken like ا and is called ألف مقصورة .
viii. The و or ي are not converted to ا when they are ع of a root in a verb of form فَعِلَ or فَعَلَ when possessed of qualificative أَفْعَل in the sense of اِفْعَلَّ & made to accord with it. In other words, when meaning shade of the verb is related to Nouns of Colors/Firm Qualities on pattern أَفْعَل & form IX اِفْعَلَّ , then there is no substitution of ا for و or ي .
Examples:
There is no substitution of ا for و or ي in the verb حَوِلَ on pattern فَعِلَ , because it has meaning shade related to أَحْوَل (squint eyed) and اِحْوَلَّ .
ix. When و is ع of اِفْتَعَلَ i.e. Form VIII with reciprocity meaning shade i.e. similar to Form VI meaning shade, then there is no substitution of ا for و .
When ي is ع of اِفْتَعَلَ i.e. Form VIII with reciprocity meaning shade i.e. similar to Form VI meaning shade, then despite this reciprocity meaning shade, still there is substitution of ا for ي .
When there is no shade of reciprocity, there is substitution of ا for both و & ي as عof اِفْتَعَلَ i.e. Form VIII.
Examples:
اِخْتَارَ with ا substituted for ي was originally اِخْتَيَرَ and has pattern اِفْتَعَلَ and root خ ي ر .
يَخْتَانُ with ا substituted for و was originally يَخْتَوِنُ and has pattern يَفْتَعِلُ and root خ و ن .
There is no substitution of ا for و in اِجْتَوَرَ because it is in the meaning shade of تَجَاوَرَ in which the و is preserved from conversion.
x. When و or ي are immediately followed by a letter entitled to transformation (which is also usually و or ي ) , mostly this immediately following letter is transformed, while و or ي coming before it are not converted. Sometimes, the reverse occurs i.e. و or ي coming before are converted while the immediately following letter is not transformed. Why the letter coming later is transformed more often, because the final is the seat of alteration and the more the letter is near the end of a word, the more entitled it is to transformation. Why both the letters are not transformed, because 2 transformations without a separative, i.e. together/back to back in a single word are mostly avoided.
Examples:
شَوَى with ا substituted for ي was originally شَوَيٌ and has pattern فَعَل and root ش و ي .
The ا at the end of this example is written ى but spoken like ا and is called ألف مقصورة .
غَوَى with ا substituted for ي was originally غَوَيَ and has pattern فَعَلَ and root غ و ي .
The ا at the end of this example is written ى but spoken like ا and is called ألف مقصورة .
مَآء with ا substituted for و and أ substituted for ه was originally مَوَه and has pattern فَعَل and its root is م و ه .This is one of the few examples where 2 transformations occurred together/back to back without a separative.
حَيَوٰة with ا substituted for و was originally حَيَوَة and has pattern فَعَلَة and root ح ي و .
The ا at the end of this example is written وٰ but spoken like ا and is one of the few examples of words written like this. Other words written like this in The Qur'an include: زَكَوٰة , صَلَوٰة and غَدَوٰة .
ءَايَة usually written آيَة with ا substituted for 1st ي was originally أَيَيَة and has pattern فَعَلَة and its root is أ ي ي .
xi. When the و or ي are ع of a root in a word, and the word ends in an augment only found in nouns and not found in verbs, then mostly there is no substitution of ا for و or ي .
Example:
سَيَلاَن with augmentative ان at the end with pattern فَعَلَان and root س ي ل has no ا substituted for ي because of the augment ان at the end peculiar to nouns.
2. When 2 أ come together and 1st is mobile e.g. أَ or إِ or أُ and 2nd is quiescent then this 2nd is changed to ا if 1st is pronounced with fatha i.e. أَ , to ي if 1st is pronounced with kasra i.e. إِ and to و if 1st is pronounced with damma i.e. أُ .
Examples:
ءَاخَر usually written آخَر with ا substituted for أْ was originally أَأْخَر and has pattern أَفْعَل and its root is أ خ ر .
ءَامَنَ usually written آمَنَ with ا substituted for أْ was originally أَأْمَنَ and has pattern أَفْعَلَ and its root is mentioned next i.e. أ م ن .
3. Quiescent أْ preceded by fatha can be converted to ا as in below example.
Example:
تَارَة with ا substituted for أْ was originally تَأْرَة and has pattern فَعْلَة and its root is ت أ ر .
Additional Notes :
For tanween in accusative case ا is added as in مُسْلِمًا which is written instead of مُسْلِمً , and for ن of emphasis, it is sometimes substituted as in لَنَسْفَعًا which is written instead of لَنَسْفَعَنْ , but still ن is spoken in both the cases.