Since you have had Arabic before, you have probably seen verbs like إتَّبَعَ , إضْطَرَّ , إزْدادَ , and إصْطَدَمَ These are all examples of form viii verbs that have assimilated the infixed ت Students are usually not pleased when they first see this phenomenon. I am sure you are not any more pleased now than you were then. At least this time around, I hope, you are less intimidated by the language and realize that many things are nowhere near as difficult as they seem.
What is happening with the above verbs is that the first radical of each of them is affecting the ت which is infixed in order to form form VIII. There are actually nine offending letters which cause changes to the ت. You do not need to memorize them, although I will list them below. All you need to be aware of is the kind of changes they make to the ت so that you will be able to identify the roots of words which incorporate these changes. For example, you see the word إدَّعى (which will not have the shadda in most texts) and wonder what the hell it is. Well, IF you know that the letter د assimilates the ت completely (I know that it is a big “if’), you will be able to guess that the word you are looking at is the form viii of the verb يَدْعو , دعا with which you fell in love some chapters ago.
The offending letters are all produced near the front of the mouth. They are either dental or emphatic. They are:
ت ث د ذ ز ص ض ط ظ
Whenever any of these letters is the first radical in a form viii verb, the ت will undergo some kind of change. Volume I of EMSA gives good examples. I will use them here.
The form I ثأَرَ becomes إثأرَ in form viii and the form I دعا becomes إدَّعى in form VIII. When these form viii words are written without the shaddas (do you think that will happen very often?) they look just like form IV verbs in the past tense. In the imperfect, they look just like Forms I, II, and IV. I wonder if context will help you decide?
The ط and the ظ also completely assimilate the infixed ت . For example, طَلَعَ is a form I meaning, among other things, “to appear.” The form IV of this verb is أَطْلَعَ meaning “to inform (someone of something).” The form viii is إطَّلَعَ meaning “to inform oneself’ or “to be informed.” All three verbs in the imperfect look like this: يطلع
The ظ works just like the ط. Fortunately, there is only one word in the language that I know of that has ظ for its first radical and also exists in form VIII. That verb is إظلمَ Look it up. I hope its meaning does not describe the way you feel.
The ذ combines with the infixed ت to form a د with a shadda. Thus, the form viii of ذكر “to remember” is إِذَّكر “to remember.” Why bother to make the form viii for this verb?
The ز turns the infixed ت into a د also. The form viii of زاد is إِزْدادَ. This is a verb which you may have seen many times.
If you wish, you may memorize the changes I have just outlined above. That is, if you have nothing else better to do. I recommend, instead, that you simply be aware of a د or ط appearing in a word where a ت should be, and that you keep in mind the form viii potential of roots that start with ط , ظ ت , ث ,د and ذ. Experience with the language will familiarize you with the very limited number of verbs in form viii which pull this sort of thing on you. The more common ones are included in your drills on form VIII.