The girl is running ___ she should miss the train.

• Lest এর পরের Clause এ Should/ might/ be বসে।
- Lest নিজেই Negative হওয়ায় এর পর আর কোনো negative শব্দ no/not/never বসে না।

• Structures:
- Lest + 1st subject + should/might + base form + complement.
- Lest + 1st subject + should be /might be +be+ V3 + complement.

• সুতরাং, নিয়মানুযায়ী সঠিক উত্তর হবে - should.
- Complete sentence: The girl is running lest she should miss the train.

• Few Examples:
- Work hard lest you should fail.
- He walked fast lest he should miss the train.
- He fled the country lest he be captured.
- Hesitant to speak out lest he be fired.

Source: Collins Dictionary, Merriam Webster Dictionary.