The article الـ has two patterns. Before moon letters, its lām is pronounced distinctly; before sun letters, it assimilates into the following consonant.
Compare الْقَمَر (audible lām) and الشَّمْس (lām absorbed into shīn). The article has its own systematic rule.
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Lām of the definite article
Idghām: Types, Complete/Incomplete Assimilation and the Definite Article
The article الـ has two patterns. Before moon letters, its lām is pronounced distinctly; before sun letters, it assimilates into the following consonant.
Compare الْقَمَر (audible lām) and الشَّمْس (lām absorbed into shīn). The article has its own systematic rule.