They worked hard enough (finish) the project on time.

• Enough (যথেষ্ট) শব্দটি adjective ও adverb হিসেবে ব্যবহৃত হয়। 

• Adverb/adjective এর পর 'enough' বসলে to-infinitive (to + verb এর base form) ব্যবহৃত হয়। 
- Structure - adverb/adjective + enough + (to+verb+....) 
- Example - He didn't run fast enough to win the race. 

• আবার,
- adjective/adverb/verb + enough + for + pronoun (me/him/her, etc) + (to+verb) এরূপেও ব্যবহৃত হয়। 
- Example - What he says is not clear enough for me to hear him clearly. 

• প্রদত্ত sentence এ 'hard' (adjective) এর পর 'enough' শব্দটি এসেছে, তাই to-infinitive (to + verb এর base form) ব্যবহৃত হবে। 
- Complete Sentence - They worked hard enough to finish the project on time.

Source: A Passage to the English Language by S.M. Zakir Hussain.