Mental capacity is the ability to make or communicate specific decisions at the time they need to be made. To have mental capacity you must fully understand the decision you need to make, why you need to make it and the likely outcome of what you decide.
It’s important to remember that needing more time to understand or communicate your decision doesn’t mean you lack mental capacity. For example, if someone has dementia, it doesn’t necessarily mean they’re unable to make any decisions for themselves. If, for whatever reason, someone is having difficulty communicating their decision, attempts should always be made to overcome these difficulties to help the person decide for themselves where possible.
If they can’t, this is where a lasting power of attorney would step in to make sure their wishes are made clear.