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There are several possible words:For useless meaning valueless, the opposite could be "treasured."For useless meaning not workable, the opposite could be "effective."For useless meaning not required, the opposite could be "necessary."For useless meaning worthless, the opposite could be "practical."For useless meaning inappropriate, the opposite could be "pertinent."For useless meaning unhelpful, the opposite could be "desirable."useful - look it up in a dicitonary and you'll see a picture of me.
For critical meaning criticizing (in a negative way), the opposite could be complimentary, praising, or laudatory. For critical meaning vital, the opposite could be secondary, unimportant, or unnecessary.
No, the additional prefixes are redundant and add nothing to the meaning.
The opposite of thin is thick.For persons, the opposite of thin may be fat, portly, or heavy.For thin meaning tenuous, the opposite could be dense.For thin meaning less than complete, the opposite could be substantial, or complete.Thick
For the meaning of assert as "to affirm" the opposite could be "deny."
There is no opposite of the noun "longevity" meaning lifespan. However, the opposite of longevity meaning endurance could be instability or ephemerality. Also, the opposite of long-lived (lasting) could be short-lived, brief, or ephemeral.
The opposite of thin is thick.For persons, the opposite of thin may be fat, portly, or heavy.For thin meaning tenuous, the opposite could be dense.For thin meaning less than complete, the opposite could be substantial, or complete.Thick
Basic
For critical meaning criticizing (in a negative way), the opposite could be complimentary, praising, or laudatory. For critical meaning vital, the opposite could be secondary, unimportant, or unnecessary.
The opposite of thin is thick.For persons, the opposite of thin may be fat, portly, or heavy.For thin meaning tenuous, the opposite could be dense.For thin meaning less than complete, the opposite could be substantial, or complete.Thick
The opposite of scenic (meaning having natural beauty) could be unsightly, ugly, or unpleasant.
The opposite of a mass battle could be single combat : a duel or bout. There is no direct opposite meaning "not" a pitched battle.