dad

ض

The Arabic letter dad can appear in four different forms: isolated, initial, medial and final.

finalmedialinitialisolated
ـضـضـضـض

When you show the letter on its own, you use the isolated form.

ض

When the letter is the first letter of a word, you use the initial form.

ضـ

For example these words:

pressure
DaghT
ضَغط
light
Daw'
ضَوء

When the letter is not the first and not the last but somewhere in the middle of a word, you use the medial form in most cases.

ـضـ

For example these words:

rage
ghaDab
غَضَب
favour
faDl
فَضل
embrace
HaDn
حَضن

When the letter is the last letter of a word, you use the final form in most cases.

ـض

For example these words:

some
ba3D
بَعض
running
rikD
رِكض
soft
ghaDD
غَضّ

Now you know that:

In most cases...
Let us look at the other cases!

The letter dad has dual joining, like most Arabic letters. It wants to join with both the letter before it and the letter after it.
But some letters has right joining. They only want to join with the letter before it (which is the letter to the right since Arabic is written from right to left).

When the letter is in the middle of a word, and follows a right joining letter, you use the initial form.

ضـ
situation
waD3
وَضع

When the letter is the last letter of a word, and follows a right joining letter, you use the isolated form.

ض
land
'arD
أَرض
disease
maraD
مَرَض
basin
HawD
حَوض

Now you know that:

That's it!
The rules you just learned can be applied to all Arabic letters with dual joining, which is most of them.

Previous letter: sad ص Next letter: tah ط