It is time we (change) our bad habits.

• It is time/It is high time:
→ It is time/It is high time থাকলে আর এর পর subject বসলে subject - এরপরের verb টি past form হয়।

→ It is time we (change) our bad habits.
- এর সঠিক verb হবে changed. 
- কারণ, It is time এর পরে subject – we এসেছে এবং তার পরে 'change' verb হিসেবে এসেছে। 
- তাই, ঐ verb 'change' এর past form 'changed' হবে।

Correct Expression: It is time we changed our bad habits.

It is time ও It is high time এর পরে subject না এসে, কোন verb আসলে ঐ verb এর Infinitive form হবে। 
যেমন: It is high time to change our attitude.

Example Sentence:
1. It is high time you started taking your health seriously and began exercising regularly.
2. It is high time we addressed the environmental issues affecting our community.

Source: A Passage To The English Language: S. M. Zakir Hussain.