In Arabic, the dual form is used to indicate two of something. 

When a noun ending in "taa marbuta" (ة) takes a suffix, the ending changes to "taa maftuha" (ت). 

For dual forms, "انِ" (aani) is added in the nominative case and "ين َ  " (ayni) in the accusative/genitive cases. 

The final "i" in "aani" and "ayni" isn't pronounced if followed by a pause. 

So, "وَلَد" (walad, boy) becomes "وَلَدَانِ" (waladāni) and "وَلَدَيْنِ" (waladayni), and "شَجَرَة" (shajarat, tree) becomes "شَجَرَتَانِ" (shajaratāni) and "شَجَرَتَيْنِ" (shajaratayni).

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