Ask two different questions: What is the relationship between the consonants? That gives identical/cognate/nearby. Does any feature of the first survive assimilation? That gives complete/incomplete.
In مَنْ يَقُولُ the retained nasal quality marks partial assimilation; in قُلْ رَبِّ the independent lām articulation does not survive, illustrating complete assimilation.
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How to distinguish complete and partial
Idghām: Types, Complete/Incomplete Assimilation and the Definite Article
Ask two different questions: What is the relationship between the consonants? That gives identical/cognate/nearby. Does any feature of the first survive assimilation? That gives complete/incomplete.
In مَنْ يَقُولُ the retained nasal quality marks partial assimilation; in قُلْ رَبِّ the independent lām articulation does not survive, illustrating complete assimilation.