past simple & past perfect

Question

What is the difference between using the simple past in two parts of the sentence and using the past simple in one part and the past perfect in the other part. For example:

"As soon as the light started changing, the birds went silent"

What does that exactly mean? Why did not we say:

"As soon as the light had started changing, the birds went silent"

In the following sentence, which one is correct: ate or had eaten>\?

When I came home, my wife and my sister ate dinner.
When I came home, my wife and my sister had eaten dinner.

Answer 1

In your second example, "had eaten" is correct.

Your first example is a special case.

Past perfect is used when an action happens before some time in the past. For example, you came home in the past, and your wife had eaten dinner before you came home.

In your first example, "as soon as" means two actions happened at the same time, not one before the other, so we wouldn't use past perfect.

Correct:

As soon as I went outside, it started raining. (It started raining at the same time I went outside.)

As soon as I got home, my wife ate dinner. (My wife ate dinner at the same time I got home.)

As soon as the light changed, the birds went silent. (The birds stopped making noise at the same time the light changed.)

Also correct:

When the light changed, the birds had already gone silent. (The birds went silent before the light changed.)

Answer 2

Two actions happened in the past. We can use the past perfect tense for the former action and the simple past tense for the later one.

When I came home, my wife and my sister had eaten dinner.

My wife and my sister had eaten dinner before I came home.

We can also use the simple past for both.

My wife and my sister ate dinner before I came home.

Two actions happened at the same time. We usually use the simple past tense (with as soon as) for both the actions.

As soon as the light started changing, the birds went silent.