Hamze (Arabic hamza) is a glottal stop, exclusive to the Arabic loanwords (more in Phonology). It is pronounced very close to the Cockney pronunciation of /t/ in the word “bottle”; e.g., شیء šey’ ‘thing, an object.’

While in Arabic hamza takes a “seat” (Arabic kursi; Persian korsi) as an alef in the initial position (i.e., ,إ ,أ), in Persian a mere alef represents an initial hamze; e.g., اثر asar (Arabic أثر aϑar) ‘impression, effect, trace.’

It must be noted that in Persian, any initial vowel or diphthong is accompanied by an alef:
اِیـ , اُو , ایـ , او , آ , اُ , اِ , اَ = a-, e-, o-, ā- (= aa-), u-, i-, ow, ey. For instance:

cloud abr ابر horse asb اسب
name esm اسم beginning ebtedā’ ابتداء
act of falling oft افت familiarity ons انس
water āb آب fire ātaš آتش
he, she u او ouch! ukh !اوخ
Iran irān ایران this in این
situation owzā’ اوضاع culmination owj اوج
also eyzan اِیضاً province eyālat ایالت

In the medial position hamze is a carrier and the procedure is as follows:

If the hamze is preceded by a fathe; that is, an /a/ (not necessarily immediately), the korsi would be an alef; e.g., رأی ray ‘opinion, a vote,’ مَسأله masale ‘a problem.’ (Occasionally some Persian writers use a notch instead, as مسئله. It is so common a mistake that the same author may use both variations.)

If the hamze is preceded by a kasre (that is, an /e/), a long [ā], or if it is in initial form (that is, in the medial position, but after و , ژ , ز , ر , ذ , د , ا ), the korsi is a notch; e.g., زائده ede ‘a process, protuberance,’ فائده ede ‘profit, gain, use,’ اِئتلاف etelāf ‘alliance, coalition,’ اِستئصال estesāl ‘extreme helplessness,’ اِستئناف estenāf ‘appeal.’

In Persian, a hamze of the section [b] type is usually normalized as یـ; for example:

فائده ~ fāyede فایده
زائده ~ zāyede زایده
استئناف ~ estināf استیناف
استئصال ~ estisāl استیصال

If hamze is preceded by a zamme (that is, an /o/), the korsi would be a و (vāv); e.g., سؤال soāl ‘question.’ Occasionally some writers with a limited orthographic knowledge add a notch before the و , as سئوال; it is important to understand that this variation is incorrect. A notch is used only in the following situation: When this hamze (ؤ) is followed by a و, the ؤ is replaced by a notch; e.g., شئون šoūn (plural of شأن šan ‘rank; dignity’).

In terminal position hamze does not take a korsi and always stands alone; e.g., ابداع ebdā’ ‘creation, innovation,’ ابتداء ebtedā’ ‘the beginning,’ املاء emlā’ ‘dictation,’ شیء šey’ ‘thing, an object.’

It must be understood that hamze is exclusive to Arabic, and words of Persian origin do not take hamze. Therefore, writing the Persian word آیین āyin ‘ritual, custom, ceremony’ for instance as آئین is incorrect.

The same is true in the case of the euphonic [y] used when the Indefinite [i] follows a long [ā], long [ū], or the silent /h/; for example:

پایی بلند pāi boland ‘long leg’, never پائی بلند
مویی زیبا mui zibā ‘beautiful hair’, never موئی زیبا
خانه یی بزرگ xānei bozorg ‘big house’, never خانه ئی بزرگ

In the case of the silent /h/, the variation ای may be—and is widely—used; as in:

خانه ای بزرگ
xānei bozorg
‘big house’
بچّه ای کوچک
baccei kucak
‘small child/kid’

In Persian it is very common to find writers who, as a result of over compensation, confuse the ئـ and the یـ, such that even such words (of Arabic origin) as رئیس rais ‘chief, principal, head master, etc.’ is occasionally written as رییس. If we understand that the hamze is exclusive to Arabic, then we would not have any problem whatsoever. Words such as غائی ,رئیس γāi ‘final,’ etc. must be written with the hamze.

Remember that hamze is a consonant, hence, in any context, especially in the ezāfe construction (possessive or adjectival), hamze must be treated as a consonant. Therefore: انشاء خوب ‘(a) good (written) composition,’ not انشای خوب and انشائی نوشتم, not انشایی نوشتم (this is yet another form of over compensation similar to writing یـ for ئـ).

In colloquial Persian, a final hamze is normally elided. In this case, the word is treated as one ending in a vowel (the vowel preceding the hamze) and in ezāfe construction, an euphonic /y/ is used with them; e.g., اعضاء a’ ‘members’ would give …اعضای a:zā-ye…, and ارتفاع ertefā’ ‘height’ would give ارتفای این درخت ertefā-ye in deraxt ‘the height of this tree.’ This, however, should not be used in written/formal material or in a well articulated and sophisticated speech.

Words may have a combination of مدّ , تشدید , همزه , تنوین ; e.g.: 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).