These are called hypogeusia and ageusia, respectively, and refer to the decrease and disappearance of the ability to detect the flavour of food.
You can use mint, basil, oregano, thyme, bay leaf, fennel, garlic, citrus juices, salt (as long as it is not contraindicated) or flavour enhancers (bullion cubes). In case of painful swallowing or acid reflux, avoid dishes with garlic and acidic foods.
This helps to release flavours and increases saliva production.
The perception of one flavour may be more compromised than another one. Try to find out which flavours are altered or diminished (acid, bitter, sweet, salty, umami) to enhance meals with the flavours that are tasted more. The following recipe can be used as an example of adapting dishes to the different basic flavours.