1. This house was built by my uncle.

Active voice
Passive voice
2. The letters were sent to Moscow yesterday.

Active voice
Passive voice
3. They don't read these books.

Active voice
Passive voice
4. I will become a famous composer one day.

Active voice
Passive voice

1. Active voice: My uncle built this house.
To transform the sentence from passive to active voice, we need to identify the subject performing the action (my uncle) and make it the subject of the sentence. Then, we use the appropriate verb (built) and add any necessary complements or objects.

2. Passive voice: Moscow was sent the letters yesterday.
To transform the sentence from active to passive voice, we need to make the object (the letters) the subject of the passive sentence. The original subject (unknown or not relevant) becomes the agent and is placed after the verb. The verb (sent) is changed to its passive form (were sent) and the preposition "to" is introduced before the new subject.

3. Active voice: These books are not read by them.
To transform the sentence from passive to active voice, we need to identify the subject performing the action (them) and make it the subject of the sentence. Then, we use the appropriate verb (read) and add any necessary complements or objects. In this case, we also introduce the negative auxiliary verb "do not" before the main verb.

4. Active voice: One day, I will become a famous composer.
The original sentence is already in active voice. It states the subject (I) and the main verb (will become) along with the complement (a famous composer). No transformation is necessary in this case.

It is important to note that the passive voice is used to focus on the object receiving the action instead of the subject performing the action. The active voice, on the other hand, emphasizes the subject performing the action. The choice between active and passive voice depends on the context and the intended focus of the sentence.