In Arabic the sun letters are, ت ث د ذ ر ز س ش ص ض ط ظ ل ن. When these letters follow the definite article Al, the "l" isn't pronounced, but the initial constant is doubled. That is why "اَلْصَّحيفة" sounds like "assahifa" and not "alssahifa." The moon letters are أ ب ج ح خ ع غ ف ق ك م ه و ي. The definite article "Al" is pronounced fully after one of these constants proceeds it. Example "القمر" (the moon, is pronounced al-Qamr).
that is correct. Sometimes the al is not pronounced exactly as written - it becomes a + stress on the first consonant of the indefinine word, so it would be assa7iifah
The rule is: it depends on the consonant letter. Some letters are called 'solar letters' (you do not pronounce the 'l' in al, and stress the letter instead) and some are lunar letters (you pronounce the al)