Enough to shows sufficiency and has a positive meaning. Note that enough goes after the adjective or adverb it modifies.

• She is old enough to do things on her own.
• He is strong enough to lift that box.
• The boy was clever enough to solve the problem.
• They are rich enough to buy just about anything.
• He ran fast enough to catch the train.
• She worked hard enough to pass the test.

The phrase ‘not enough to’ shows insufficiency.

• She isn’t old enough to marry.
• He is not strong enough to lift that box.
• The boy was not clever enough to solve the problem.
• We aren’t rich enough to buy a car.
• He didn’t run fast enough to catch the train.
• She didn’t work hard enough to pass the test.

Notes

Enough goes after the adjective or adverb it modifies.

• She is old enough to have grandchildren. (NOT She is enough old to have grandchildren.)
• He is intelligent enough to solve that problem. (NOT He is enough intelligent to solve that problem.)

Enough can also modify nouns and then it goes before them.

• We have bought enough eggs. (NOT We have bought eggs enough.)
• She earns enough money to lead a comfortable life. (NOT She earns money enough to lead a comfortable life.)

Note the difference in the meaning when the position of enough changes.

• We haven’t got big enough tables. (= We need bigger tables. The tables that we have at the moment aren’t big enough.)
• We haven’t got enough big tables. (= Our tables are big enough, but we haven’t got enough of them. We need more big tables.)