أ as Augmentative Letter.
The أ can be judged to be augmentative in following cases:
1. When أ is 1st letter of a word, and is followed by 3 root letters. This is true for both verbs and nouns.
أَفْعِلُ - أَجْلِسُ - ج ل س
أَفْعَلَ - أَنْزَلَ - ن ز ل
أَفْعَل - أَكْبَر - ك ب ر
Note: Some cases to be careful about. When 3 letters follow أ but one of them can also be an augmentative letter, in such cases, أ may be an augmentative letter or root letter. In all below cases أ is root letter.
فُعْلَى - أُنْثَى - أ ن ث
فِعْلِيّ - إِنْسِيّ - أ ن س
فَاعَلَ - آمَنَ - أ م ن
2. When أ is 1st letter of a word, and is followed by 2 root letters, then أ is also a root letter. Note in all the following, it is followed by 2 root letters OR 2 root letters + augmentative letters, i.e. some other letter is augmentative, not أ
فِعْلَان - إِنْسَان - أ ن س
فِعَال - إِمَام - أ م م
فَعَال - أَثَاث - أ ث ث
فَعِيل - أَصِيل - أ ص ل
فَعْل - أَرْض - أ ر ض
فَعَل - أَجَل - أ ج ل
3. Some other cases of أ as 1st letter and augmentative letter are given below, these can be studied to understand its role better.
أَفْعَلَة - أَرْبَعَة - ر ب ع
أَفْعَلِيّ - أَعْجَمِيّ - ع ج م
أُخْدُود - أُفْعُول - خ د د
إِعْصَار - إِفْعَال - ع ص ر
4. Most common place for أ as augmentative letter is at beginning of word, i.e. as 1st letter of a word, but there are few cases when it is augmentative at other location also. See below examples to understand better. In 1st example, it is used in plural. In 2nd and 3rd the ٱ is augmentative as 1st letter while the أ is augmentative and comes after (1 augmentative letter) and 2 root letters and is followed by 1 root letter (which is doubled)
فَعَائِل - شَمَآئِل - ش م ل
ٱفْعَأَلَّ - ٱطْمَأَنَّ - ط م ن
ٱفْعَأَلَّتْ - ٱشْمَأَزَّتْ - ش م ز
5. When it is the last letter of a word and occurs immediately after augmentative ا and before this ا there are 3 root letters.
فَعْلَآء - نَعْمَآء - ن ع م
فُعْلَآء - عُشْرَآء - ع ش ر
فِعْلِيَآء - كِبْرِيَآء - ك ب ر
فُعَلَآء - كُبَرَآء - ك ب ر