If something is inadequate, there is not enough of it or it is not good enough.
deficient
Supplies of food and medicines are inadequate.
食品和药物供应不足。
The problem goes far beyond inadequate staffing.
问题远远不止于人员不足。
If someone feels inadequate, they feel that they do not have the qualities and abilities necessary to do something or to cope with life in general.
adequate
I still feel inadequate, useless and mixed up.
我仍旧感到能力不足,毫无用处,一团混乱。