When the definite article «ال» (“the”) comes before a word, Arabic letters split into two types: sun letters, whose «ل» is written but not pronounced, and moon letters, whose «ل» is both written and pronounced.
They're called “sun” letters because the word «الشمس» (“the sun”) is itself an example: we write the «ل» but don't pronounce it — instead the next letter is doubled, so we say “ash-shams.” Other sun letters include: ت ث د ذ ر ز س ش ص ض ط ظ ل ن.
The word «القمر» (“the moon”) is an example of a moon letter: we write and clearly pronounce the «ل», saying “al-qamar.” Other moon letters include: أ ب ج ح خ ع غ ف ق ك م ه و ي. Try having your child listen for the «ل» sound in words like «الكتاب» and «الشمس» to discover the difference themselves.
