The alif isn't actually a sound in itself. It's a marker for a vowel.
At the beginning of a word it can be a marker for any of the short vowels (a, i u) with a hamza.
The Hamza (glottal stop may or may not be written)
If it's at the bottom of the alif, it' an 'i'
إ
If at the top, it can be either 'a' or 'u'
أ
In the middle or the end of a word, it indicates a 'long a' or 'aa'
Although in some cases at the end of a verb in the third person plural, it's not pronounced at all.
Brian from Brussels
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