What is the difference between "merits"and"strengths" and what is the difference between "demerits" and "weaknesses"?
The noun merit can mean
often plural a deserving or commendable quality or act: judge him on his merits
The verb form has a similar tone
to be worthy of; deserve
Both forms connote some praiseworthy activity or effort.
Strength, by contrast is a more passive characteristic
something that is regarded as being beneficial or a source of power: their chief strength is technology
Demerit even more clearly suggests action, albeit negative
- something, esp conduct, that deserves censure
- (US & Canadian) a mark given against a person for failure or misconduct, esp in schools or the armed forces
- a fault or disadvantage
Weakness also has a more passive connotation
- the state or quality of being weak
- a deficiency or failing, as in a person's character
- a self-indulgent fondness or liking a weakness for chocolates
While there is some overlap in both pairs, there are reasons to choose one over the other.
Willpower was her strength and her exercise of that determination was among her principle merits. His weakness was a kind heart. He often acts on that and, in business, that can be a demerit.
To me, merits/demerits are given based on specific activities (you received 1 merit badge when your answer is accepted on Stack Exchange) and strengths/weaknesses are inferred based on the merits/demerits one earns (her many badges for accepted English Language Stack Exchange answers show that English usage is one of her strengths, while his many downvoted answers on calculus in the Math StackExchange show that integrals is a weakness for him.).
Another way to see it is that merits/demerits can be awarded objectively (that is, without context) while strengths/weaknesses are subjective (and therefore require context to be awarded).