The definine Article (particle) in Arabic (الـ)  . (الـ)  is pre-fixed to nouns only, i.e. (الـ) is connected to the nouns it makes definite.  When (الـ) is prefixed to a noun, the first letter in this noun determines how or whether it is pronounced. The letters of the alphabet fall into two groups in terms of their relation to (الـ). The occurance of any of the moon letters as the first letter in the noun word requires the pronunciation  of (الـ), while the occurance of any of the sun letters requires omitting the (ل) sound and doubling the relevant sun letter.

The set of sun letters is called like that because the (ل) is omitted in the word (the sun)  الشّمس; it is pronounced ash-shams.
The set of moon letters is called so because the (ل) is very clear in the word (the moon) القمر alqamar/strong>.
In the illustarion below, an explanation how we articulate the definite article when it is pre-fixed to a noun starting with a sun letter. The doubling of a letter requires shaddah which means the reoccurance of the same consonant; the first with sukuun and the second with either dhammah, fatha, or kasra. The double consonant will be writeen as shown in the in the illustrating below.
Here are some examples of words with (الـ) and a noun starting with a sun letter,

Here are some examples of words with (الـ) and a noun starting with a moon letter,

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