strengthen enhance increas既分別

strengthen - 加強, 增強 (跟體能,外觀有關)
We should stengthen our body by doing lots of exercises.
我們應做多些運動來增強我們的體魄.

enhance - 提高(跟quality有關,　或解作提高一個level.)
Reading could enhance the writing skills of students.
閱讀能提高學生的寫作能力.

increase - 增加, (通常用在數量)
The number of people in Hong Kong is increasing.
香港人數在增加.
(這句其實能簡化.
Hong Kong population is growing.
香港人口在增長.)