If they had taken the earlier flight, they could arrive on time.
Change the underlined words appropriately.

• If clause এ যদি (had + V₃) হয়, তবে পরবর্তী clause এ would/could/might + have + V₃ + extension ব্যবহৃত হয়।
- এটাকে Third conditional বলা হয়।
- আবার Conditional sentence- এ Had + sub + V₃ + extension, এভাবে কোন Sentence শুরু হলে সেটিও Perfect conditional/ 3rd conditional হয়।
Example:
- If you had not helped me, I would have failed the examination.
- If you had come earlier, you would have found a good seat.
- If you had started earlier, you would not have missed the train.
- Had I been a king, I would have helped the poor.

So, Complete Sentence - If they had taken the earlier flight, they could have arrived on time.
(Meaning: They didn't take the earlier flight, and as a result, they didn't arrive on time.)

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