Key Concepts:
1. 'Must' and 'should' are modal verbs used to express obligation, necessity, or advice in English grammar.
2. 'Must' is used to indicate a strong obligation or necessity, while 'should' suggests a recommendation or advice.
3. Both 'must' and 'should' are followed by the base form of the verb.

Usage Examples:
1. You must complete your homework before going out to play. (तुम्हें खेलने से पहले अपना होमवर्क पूरा करना चाहिए।)
2. We should eat healthy food for a balanced diet. (हमें संतुलित आहार के लिए स्वस्थ भोजन करना चाहिए।)
3. She must arrive on time for the meeting. (उसे मीटिंग के लिए समय पर पहुंचना चाहिए।)
4. You should apologize for your mistake. (तुम्हें अपनी गलती के लिए माफी मांगनी चाहिए।)

Key Mistakes:
1. Confusing the usage of 'must' and 'should'. Remember that 'must' indicates a stronger obligation or necessity, while 'should' suggests a recommendation or advice.
2. Forgetting to use the base form of the verb after 'must' or 'should'. Always use the base form of the verb after these modal verbs.

Learning and understanding the usage of 'must' and 'should' in English grammar is essential for effective communication. By using these modal verbs correctly, you can express obligations, necessities, and recommendations clearly. Practice using 'must' and 'should' in different contexts to improve your English language skills.