Active voice and passive voice are two different ways of expressing the same idea but with a difference in emphasis on the subject and object of a sentence.

**Active Voice:**
- In active voice, the subject of the sentence performs the action denoted by the verb.
- The structure of an active voice sentence is typically: **Subject + Verb + Object**.
- Example: "The cat (subject) chased (verb) the mouse (object)."
- In this sentence, the cat is performing the action of chasing, making it the subject and the focus of the sentence.

**Passive Voice:**
- In passive voice, the object of the action becomes the subject of the sentence.
- The structure of a passive voice sentence is typically: **Object of the action + Verb (in passive form) + Subject**.
- Example: "The mouse (subject) was chased (verb in passive form) by the cat (agent)."
- Here, the mouse is the one being chased (object in active voice), but in passive voice, it becomes the subject. The agent (the cat) is mentioned with "by" but is not always required.

### Key Differences:

1. **Focus:**
- Active voice emphasizes the subject that performs the action.
- Passive voice emphasizes the action itself or the recipient of the action.

2. **Structure:**
- Active voice follows a straightforward Subject-Verb-Object structure.
- Passive voice rearranges the structure to Object-Verb-Subject (with or without mentioning the agent).

3. **Verb Forms:**
- Active voice uses the base form of the verb.
- Passive voice uses a form of "to be" (is, am, are, was, were) plus the past participle form of the main verb.

### When to Use Each:

- **Active Voice:**
- Use active voice to clearly state who or what is performing the action.
- It is more direct and concise, making writing clearer and stronger.

- **Passive Voice:**
- Use passive voice when the focus is on the action rather than the subject.
- It can be used to emphasize the object or the result of an action, especially when the doer of the action is unknown, unimportant, or obvious from the context.

### Example Comparison:

- **Active Voice:** "The chef (subject) prepared (verb) a delicious meal (object)."
- **Passive Voice:** "A delicious meal (subject) was prepared (passive verb) by the chef (agent)."

In summary, understanding when and how to use active and passive voice can improve the clarity, emphasis, and style of your writing, depending on what you want to emphasize in your sentences.