sin

س

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

finalmedialinitialisolated
ـسـسـسـس

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

س

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

سـ

For example these words:

arrow
sahm
سَهم
secret
sirr
سِرّ
traveling
safar
سَفَر

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.

ـسـ

For example these words:

body
jasad
جَسَد
9
tis3
تِسع
department
qism
قِسم

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

ـس

For example these words:

bag
kiis
كِيس
5
khams
خَمس
sex
jins
جِنس

Now you know that:

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

The letter sin 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.

سـ
drawing
rasm
رَسم
lion
'asad
أَسَد
centre
wasaT
وَسَط

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

س
cup
ka's
كَأس
bow
qaws
قَوس
head
ra's
رَأس

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.

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