in English is:

effectiveness

,

efficacy

Mutations

Soft:(no mutation)
Nasal:(no mutation)
Aspirate:(no mutation)

Use

yr effeithiolrwydd hwn
fy effeithiolrwydd i
dy effeithiolrwydd di
ei effeithiolrwydd ef/o
ei heffeithiolrwydd hi
ein heffeithiolrwydd ni
eich effeithiolrwydd chi
eu heffeithiolrwydd nhw/hwy
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