"They play football." Transform it into Imperative.

Assertive Sentence: They play football.
Imperative Sentence: Let them play football.

Assertive Sentence এর subject যদি First person বা Third person হয়, তবে তাকে Imperative করার নিয়ম:

Structure: Let + প্রদত্ত subject টির objective form + প্রদত্ত sentence এর মূল verb থেকে শেষ পর্যন্ত।

More Examples:
Assertive Sentence: He plays football.
Imperative Sentence: Let him play football.

Assertive Sentence: We should go out.
Imperative Sentence: Let us go out.

Source: Advanced Learner's English Grammar & Composition by Chowdhury & Hossain.