What is the difference between these two sentences: 1. He would dance well when he was 10 years old. 2. He had been dancing well when he was 10 years old.
4 Answers
Your first sentence strikes my ear as marginal. When using would to refer to a regular practice in the past, modifiers typically describe aspects of the practice not judgments made about it, like well.
He would dance on his hind legs when he was a puppy but we cannot get him to do any tricks now that he is an older dog.
Your second sentence is also marginal, since with the past perfect continuous we need some sort of justification for the past perfect tense:
He had been dancing well for almost half his life when he was ten years old, having begun at the age of five.
They're both very odd things to say as sentences on their own, but...
Would for things in the past is used for things in the future of when you are talking about. So, you might say "He would dance well when he was ten years old, but when he was five he moved clumsily."
Had been dancing is past perfect progressive. The "past perfect" bit means that it is in the past of when you are talking about. He had danced would be past perfect without being progressive, and "he had danced well when he was ten years old, but by fifteen he refused to dance" would be a perfectly good sentence. However, "had been dancing" requires some more to justify the progressive, such as "he had been dancing well for several years when he was ten years old". At that point, all the dancing is in the past from the "when he was ten years old", so you don't need anything else for it to be in the past from. Past progressive, "was dancing", is used to say something about how the person was dancing at the time, or just to say that they were generally doing something - "at age ten, he was dancing" suggests that he was inclined to dance from time to time, while "at age ten, he was dancing well" suggests that when he danced at that age, he danced well.
"Would" in this context indicates an ongoing action. In other words, as a 10-year-old, he always danced well. "Had been" implies up until a certain point in the past after the action (in this case, dancing). I would expect sentence 2 to be followed by something like "But that year, he became self-conscious and stopped being able to dance like he used to." You need a second past event that the "had been" (past perfect) comes before.
Habitual Aspect: https://en.wikipedia.org/wiki/English_markers_of_habitual_aspect
This is formed from two parts: would + past.
In English “would” is a modal verb so it must occur with another verb (though the other verb might merely be implied—as in German).
Would + verb + description of past
The meaning is that the verb (dancing) is something that happened in a regular pattern in the past.
Plusquamperfect: https://en.wikipedia.org/wiki/Uses_of_English_verb_forms#Past_perfect_progressive
In the past an action occurred. This action (dancing) happened before some other action but we haven’t said what the second action was. We also don’t know for sure when the dancing ended. It ended before or after or during the other action. You will likely need a preposition or another sentence to completely explain the sequence of events.
When + other action—> possible end of dancing when other action starts.
Until + other action —> dancing ends at start of other action.
When + other action. Conjunction/adverb + continuation of dance/dancing —> Dancing continues after other action/event.
