Question: Normally we say "Assalam o Alaikum", but my sir said that for women we need to say "Assalam o Alaikuna". Is that right?

Answer: 
In regards to greeting in Arabic (for females):

According to Arabic grammar, KOM is used for men, and Kon (Konna) is used for women. As a result if you’d like to use the correct grammatical shape of it, you ought to say “Assalam o Alaikon (Konna) which is written like the following phrase:

> 
> السلام علیکن
> 
> 
> 

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Of course, there is another nice point, and it is related to singular and plural nouns. Actually “Kon” is related to plural noun (according to the correct Arabic grammar).

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But is the singular noun it is writes:

> 
> السلام علیک / Assalam o Alaike (For female)
> 
> 
>
بسم الله الرحمن الرحیم

Kum (کم) means plural you (‘men’ or 'men & women')
Kuna (کنَّ) means plural you (only women)

Yet in conversations it is common to say Alaikum, however if we are to speak with eloquence to an only women group, then say Alaikuna is more correct yet uncommon, especially in non-Arab dialogues.