The Definite Hamzah أ - This letter is clearly pronounced
The Connecting Hamzah آ - This letter has a sukun and it is not pronounced when it is in the middle of a sentence. When it is the first letter to be pronounced, it changes to a definite Hamzah, this is because we cannot begin the pronunciation of a word with a letter that has sukun in Arabic.
The rule of thumb to follow when we have to convert it to a definite hamzah is we pronounce it with a dammah if the third letter in the word has a dammah, otherwise we pronounce it with a kasra.
Examples are آخرُج -> اُخرُج , آعلَموا -> اِعلَموا
When it is the first letter of a word that begins with ال of definiteness, then we pronounce it with a fatha.
Example is آلعليم becomes/is pronounced as أَلعليم
When it comes in one of the following seven nouns, it is pronounced with a kasra. The seven nouns are as follows (ابن - اسم - ابنة - امرؤ - امرأة - اثنان - اثنتان). Also, when it is the first letter of the 5-letter or 6-letter verbs and their noun (root - مصدر) forms, it is pronounced with a Kasra.
Examples are [ابتغى - ابتغاء] [استكبر - استكبار].