Questions & Answers about تسمية الشوارع شيء مهم جدا.
Why does تسمية end with a taa marbuuta (ة) instead of following the regular Form II verbal noun pattern?
The word تسمية comes from a defective root (a root ending in a weak letter, in this case, س-م-ي). For Form II verbs with defective roots like سمّى (to name), the standard verbal noun pattern تفعيل (taf'eel) changes to تفعلة (taf'ila). This is why we get تسمية (tasmiya) instead of a word ending in a yaa.
How do I pronounce the end of تسمية in this sentence?
Because تسمية الشوارع (naming [of] the streets) is an idafa (possession phrase), the taa marbuuta (ة) on the first word must be pronounced as a 't'. You say 'tasmiyat ash-shawaari', not 'tasmiya ash-shawaari'. As the first word of an idafa, it also cannot take the definite article ال (al-) or end in an 'n' sound (tanween).
Where is the word 'is' in this sentence?
This is a nominal sentence, which doesn't use a verb for 'is' or 'are' in the present tense. The subject is the idafa phrase تسمية الشوارع (naming the streets), and the predicate (the part giving information about the subject) is شيء مهم جدا (a very important thing). The 'is' is naturally understood to be between these two chunks.
الشوارع (streets) is a non-human plural, so shouldn't its adjective be feminine singular? Why is مهم masculine?