Tanvin is an adverbial ending, which may be compared with the English adverbial suffix –ly in such words as ‘seriously,’ ‘truly,’ ‘really,’ etc. Just like hamze, tanvin is also exclusive to the Arabic loanwords.

In Arabic, tanvin has two main functions: 1) Indefinite marker, as in کتابٌ (nominative), کتابً (accusative), کتابٍ (genitive) ‘a book’; 2) adverbial ending, as in مثلاً ‘for instance, for example.’ In all these cases the ending is pronounced –an, hence the word tanvin ‘n-ization’—i.e., making sound like an “n.” In Persian, the usage of the tanvin as the indefinite marker is not exercised; that is to say, in Persian tanvin is exclusively an adverbial ending.

In its original Arabic usage, tanvin has different forms, such as ـاً , ـا , as in غالباً , غالبا γāliban ‘predominantly,’ and ـة , as in نسبةً nisbatan ‘relatively’ (variably transcribed as نسبة), etc.

In Persian tanvin is orthographically normalized as ـاً ; although occasionally some members of the older generation of scholars transcribe the word نسبة in its original Arabic form, and not as the Persian form نسبتاً . Writing غالباً as غالبا is also common among them—all forms are correct (although, because of the omition of the [ _ً ], sometimes tanvin is erroneously pronounced as a final [–ā] and not as [–an]; as in, ابداً abadan ~ ابدا abadā ‘not at all, under no circumstances,’ aslan ~ aslā ‘in principle, essentially,’ etc.

Words may have a combination of مدّ , تشدید , همزه , تنوین ; as in: اوّلاً , in which the و is mošaddad (‘having tašdid’; i.e., ‘geminated’), and the terminal alef has tanvin; تأثّر , in which there is a combination of hamze and tašdid, etc.

همزه and تنوین are exclusive to Arabic and they may not be used in words of Persian origin. Hence, the erroneous, rather illiterate and unsophisticated usage of such words as دوماً ‘secondly,’ with the Persian number دو, is merely based on analogy with اوّلاً—a word of Arabic origin. In this case, either the Arabic adverbs رابعاً , ثالثاً , ثانیاً , must be used or, otherwise, the Persian numbers must be used as follows: نخست naxost, noxost ‘first, firstly’, دوم dovom ‘second, secondly’, سوم sevom ‘third, thirdly,’ چهارم cahārom ‘fourth, fourthly,’ etc.

Also, bear in mind that the و in Persian numbers دوم dovom and سوم sevom must not be geminated. Therefore, the common pronunciation of these words as دوّم dovvom and سوّم sevvom is only on analogy with the geminated و in the Arabic اوّل awwal ‘first’ and اوّلاً awwalan ‘firstly’ (Persian avval, avvalan) (that is to say, dovom and sevom, without a geminated [v] are the correct pronunciations).