He ______ the mistakes he made in his youth.

সঠিক উত্তর: ক) cannot but regret.
• "Cannot but" এর পর verb এর base form (regret) বসে।

Complete sentence: He cannot but regret the mistakes he made in his youth.

• Cannot but এবং Cannot help but এরপর verb এর base form বসে।
- এরপর কোন infinitive বা verb এর সাথে '-s/es' যুক্ত হয় না।

Example:
- She cannot but admire his dedication to work.
- We cannot help but appreciate the beauty of nature.

• কিন্তু,
• Can't help এরপর verb + ing হয়।
Example:
I can't help feeling nervous before an exam.