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- The graph shows how house prices have risen since 1980. ( Oxford dictionary, entry: graph)
Does sentence 1 necessarily mean the starting point of rising is 1980？ I think it could be applied to the situation where the price didn't rise until, for example, 2000. Just like,
- They moved since you left.
It says they moved at a unspecified time after you left. It doesn't have to be immediately after you left.
What do you think? Thanks.
The first sentence doesn't necessarily mean that either the rise or the graph starts in 1980. It just means that you can see from the graph how much the prices have risen since 1980.
But, a presenter will usually choose a time span for a graph that highlights what they want to talk about.
For the second sentence, if you mean that they moved at an unspecified time after you left, and the point is that you won't find them there any more, it would usually be said:
They have moved since you left.
If the point is to specify who moved first, you might say
They moved after you left.
and that could refer to any time after.
If you mean immediately, you could say
They moved just after you left.
Sentence 1 says the starting point of the graph is (at least) 1980. It actually doesn't say when, or how prices rose (the graph shows that). Compare with the similar construction "The graph shows if prices have risen since 1980."
But in context, and thinking about prices more than about grammar - I'd expect a graph that goes "up" in a wiggly line.
Generally, perfective sentences with "since" mean "from that point to the present day". So if one says "I've played tennis since 1980", you would understand "I started in 1980 and still play now".
The sentence is ambiguous because of the various meanings of "how", several of which could apply naturally to this sentence.
If "how" means "that" or "the fact that", then the sentence means house prices have risen starting in 1980:
The graph shows (the fact) that house prices have risen since 1980.
But if "how" means "the way in which" or "the degree to which", then it's unclear when house prices started rising, and it's even possible they haven't risen at all since 1980:
The graph shows the degree to which house prices have risen since 1980.
The difference between the two sentences is that the first one is talking about a process that took place over a period of time (prices changing), while the second is talking about something that happened at a single time (someone moving).
When referring to a continuous process with details shown during a time period, "since" usually refers to the beginning of the period being described. If you show a graph with prices over a period of time, it would be unusual and confusing to say "since 1980" if the graph starts at 1990. While the start time given could be approximate (e.g. "since the 1980's"), there's no good reason to say a different time than the graph shows.
But when talking about a single event, "since" merely refers to some notable time before the event. In your second example, the time when someone left is an important starting time for a period, and the other event "they moved" happened at some time after that.
Note that even when you're talking about a continuous process, the first interpretation only applies if you're actually giving details. You can also make approximate statements like
Home prices have increased significantly since the 20th century.
