Meaning: The frog had already left and was no longer there when the dog woke up.
Choice of Words:Gone means no longer present, and by then means at that time.
Alternative Expressions
The frog had been gone for a long time by then.
Meaning: This means that the frog had been gone for a long time by the time the speaker was talking about it. It's like saying, The frog had been gone for a long time before I got there.
Choice of Words: The frog is a small, green animal that lives in water or near water. Had been means that something was in a certain state or condition in the past. Gone means that something is no longer present. For a long time means that something has been gone for a long time. By then means that something happened before a certain time.
By the time the speaker was talking about it, the frog had been gone for a long time.
Meaning: This means that the frog had been gone for a long time by the time the speaker was talking about it. It's like saying, The frog had been gone for a long time before I got there.
Choice of Words: The speaker is the person who is talking. Talking about it means that the speaker is discussing or mentioning something. By the time means that something happened before a certain time. Had been means that something was in a certain state or condition in the past. Gone means that something is no longer present. For a long time means that something has been gone for a long time.