What is the most important thing to consider in communication with the mentally disabled?

You’re welcome.

Thx!! Very helpful.

Understand that not all people with intellectual disabilities have the same level of cognitive understanding and often may co-present with other types of disabilities, and no single method will be applicable to all groups.
For me, this is the most important thing as it will inform the type of approach taken, and then people interacting with this population can fall back on training specific to the specific type of impairment. Autism, intellectual impairment because of stroke and Down's Syndrome may require different types of communication skills. The major universal thing will be additional time required to communicate with a patient with an intellectual disability than one without.