Substitution can be recognized by :
1. Usage of original letter instead of substitute letter in some of the variations of the word containing the substitute. This return (reversion) to original letter in a word or words can be:
i. Always (invariably) e.g. plural of a word always with original letter.
OR
ii. Mostly (prevalently) e.g. form of the word with original used more frequently than form of the word with the substitute letter. By rarity (paucity) of usage of word with substitute instead of the original letter.
The word with the original letter and with the substitute letter are synonyms
Example:
دَنَانِير plural of دِينَار (Islamic gold coin) which is originally دِنَّار and its root is د ن ر and ي is substitute for the 1st ن in the singular, while in the plural, the ن is restored.
And if this reversion is not established, then these are 2 original forms with different root letter in 1 location of the root. None is substitute for the other, both are different roots with 1 root letter different in both. All variations occur with both letters and none of the 2 can be proven to be substitute for the other.
2. When 1 letter is contained in a derivative of a root, while a different letter in all the other derivatives of the root or paradigms of its derivation, then that letter in the word is a substitute.
Example:
مُهَيْمِن (Controller, Watcher over, Guardian over, Granter of Security/Safety/Protection) of pattern مُفَيْعِل from root أ م ن with ه substituted for أ . Only this word and its related words like هَيْمَنَ and مُهَيْمَن etc. are used with ه while almost all other words from this root are used with أ . Proving ه is substitute for أ .
3. When the word containing the substitute is derived from another word in which the original letter was an augmentative letter, then that letter is a substitute for an augmentative letter.
Example:
ضُوَيْرِب (small/tiny striker) which is diminutive of ضَارِب (striker) with root ض ر ب has و substituted for augmentative ا .
4. Sometimes original letter appears in derivative while in the original form substitute is used.
Example:
مَآء (water) is original form which has substitute letters while its derived form i.e. diminutive مُوَيْه has original root letters which are م و ه , while the original form has substituted (via conversion) ا for و and substituted أ for ه . The diminutive usually restores original root letters in it.
Do you see importance of this concept now? You can't even know water's original root letters without understanding concept of substitution. Is it an advanced topic or one of the basics?
5. When an unknown pattern will be there if a letter in the word is not judged to be a substitute.
Example:
اِصْطَبَرَ (he took/acquired patience) which is originally اِصْتَبَرَ has ط substituted for ت and root is ص ب ر . This is easily identifiable as there is no such pattern اِفْطَعَلَ in Arabic.
Substitution is used for various reasons which may include:
1. Lightening the pronunciation of a word.
2. Conformity of the letters, and their approximation in : outlet or qualities
InshaALLAH more details to be discussed in coming posts.