1. In the paradigms (ʾabwāb) that begin with a ت (تَفَعُّل، تَفَاعُل، تَفَعْلُل), in the imperfect tense form (muḍāriʿ), in all the 2^nd person and 3^rd person feminine singular & dual conjugations, one of the two tāʾs[1] can be dropped:
   e.g. تَتَقَاتَلُ → تَقَاتَلُ
2. In the paradigm (bāb) افْتِعَال when the ف is either a ص, ض, ط or ظ, (ḥurūf al-ʾiṭbāq), the ت is changed to a ط.
    a. This is done because of the difficulty in articulating the ت after these letters, so ط was chosen due to its closeness in maḵraj to ت. This is only found through the practice of the Arabs. There is no rule. (e.g. اصتلح to اصطلح). The change to ط is to retain the ṣafīr (whistling)[2] of ص and ʾistiṭālaħ (elongation)[3] of ض.
    b. Doing ʾidḡām of the ص and ض with the ط is not allowed generally as the letters of ṣafīr not letting ʾidḡām with letters other than itself. The same applies with ض as it has ʾistiṭālaħ and it doesn’t do ʾidḡām with letters close to it.
   Some do say you can (e.g اصطلح to اصَّلح and اضطرب to اضَّرب). The ط changes to a ص or ض. This contradicts the usual rule of ʾidḡām.
   It is also weak to say اطَّعج from اضطعج.اظتلم has three options when it comes to changes:
    c. اظطلم
     اطّلم – as is done analogously
     اظّلم
    d. If the ف is a ط then there is natural ʾidḡām (اطّرد).
3. In the paradigm (bāb) افْتِعَال, when the ف is either a د, ذ, or ز, the ت changes to a د because these words have a close maḵraj to ت. ت has hams (whispering), whereas the other letters have jahr (sound), and those together don’t work; hence د being close in maḵraj and being a letter of jahr allows it to take the place of ت.
    a. When the ف is a ذ, you have three options:
     i. اذْدَكَر
     ii. اذّكر
     iii. ادّكر
    b. When the ف is a ز, you have two options:
     i. ازدجر
     ii. ازّجر – this is done due to the lack of quality of ṣafīr when the د is present.
4. If the ف is a ث, then the ث changes to a ت. (e.g. اثْتَغَرَ to اتَّغَرَ).
5. In the ʾabwāb تَفَعُّل، تَفَاعُل, if the ف is one of these letters:
   ت، ث، د، ذ، ز، س، ش، ص، ض، ط، ظ
   The following steps can take place:
    a. Do ʾidḡām of the letter and ت: تَثَاقَلَ à ثَّاقَلَ and تَطَهَّرَ → طَّهَّرَ.
    b. Add hamzaħ al-waṣl to be able to pronounce the word: اثَّاقَلَ and اطَّهَّرَ.
6. In the bāb افْتِعَال, if the ع is one of the eleven letters above, then it is permissible to:
    a. Give a sākin to the ت letter: اخْتَصَمَ → اخْتْصَمَ
    b. Due to ʾijtimāʿ as-sākinayn, give a kasraħ to the ف letter: خِتْصَمَ
    c. Do ʾidḡām of the ت and the ع letter: خِتْصَمَ à خِصَّمَ
7. In nūn ṯaqīlaħ, in the dual forms, the reason the ن gains a kasraħ is to resemble the dual in your normal muḍāriʿ.
8. In the feminine plural for nūn ṯaqīlaħ, al-ʾalif al-fāriqaħ is placed between the nūns to separate them and then the ن gains a kasraħ like the duals to follow that pattern.
9. In the plural masculine, second person singular feminine forms, the madd is dropped and not in the dual forms because:
    a. The kasraħ and ḍammaħ indicate to a ي or و whereas ا can have a fatḥaħ but then it would resemble the mufradāt, hence to distinguish the ا is retained.

It is difficult to do madd with sākin with a kasraħ or ḍa

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[1] There is a difference of opinion on whether it is the first tāʾ that is dropped or the second. The Baṣrīyyaħ say that the second one is dropped because the letter of muḍāriʿ should not be dropped to retain the tense. The Kūfīyyaħ say the first is dropped because the second is part of the word itself to retain muṭāwaʿaħ (where the object has accepted the effect of the doer). The dropping is only done in the active form and not the passive form.

[2] ص is from them.

[3] Which is the raising of the tongue.