Expressing Ability and Obligation: Can, Could, Be able to, Must, Have to – 1st Year Secondary School – Algerian Curriculum

Introduction

Modal verbs are auxiliary verbs that express ability, obligation, permission, and possibility. This lesson focuses on expressing ability (can, could, be able to) and obligation (must, have to).

Expressing Ability

Can (Present)

Subject + can + base verb. I can speak English. Negative: can’t.

Could (Past)

Subject + could + base verb. When I was young, I could run fast. Negative: couldn’t.

Be able to (All Tenses)

Present: am/is/are able to. Past: was/were able to. Future: will be able to. Use was/were able to for specific past achievements.

Expressing Obligation

Must (Internal)

Subject + must + base verb. I must study. Negative (prohibition): mustn’t.

Have to (External)

Subject + have/has to + base verb. Students have to wear uniforms. Negative (no obligation): don’t have to.

Key Differences

Must = internal, no past form, mustn’t = forbidden. Have to = external, had to in past, don’t have to = not necessary.

Exercises

Fill in: can, could, will be able to, was able to

1. I ___ speak two languages. (can)
2. She ___ play chess at six. (could)
3. They ___ come next week. (will be able to)
4. He ___ finish the race. (was able to)

Choose: must, mustn’t, have to, don’t have to

1. You ___ brush your teeth. (must)
2. Students ___ run in hallways. (mustn’t)
3. We ___ pay taxes. (have to)
4. You ___ bring your computer. (don’t have to)

Summary

Use can for present ability, could for past ability, be able to for all tenses. Use must for internal obligation, have to for external. Mustn’t = prohibited, don’t have to = not necessary.

📍 دروس مشابهة:

• Present Simple vs Present Continuous
• Conditional Sentences