What is the difference between these two sentences:-
The traffic police has blocked the road for heavy vehicles since last Friday
It has been raining heavily since morning.
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The continuous or progressive aspect expresses an action in progress, and the perfect verb form expresses a past occurrence with present relevance. When combined, they express a past action that continues into the present.
The first sentence implies that the rain has fallen all day long, from morning until the present. In contrast, the second sentence only implies that the rain fell at some time between this morning and now. Furthermore, the first sentence suggests that it's still raining, whereas the second suggests that it stopped some time before now.
However, English isn't as strict about verb forms as some languages, and there are exceptions to the general rules. We sometimes use the perfect progressive for completed actions, as long as they finished – or were interrupted – in the very recent past. Likewise, there's no rule that present perfect always indicates a complete action; we sometimes use it for ongoing or habitual actions too. In those cases, you must consider context to divine the meaning.
This ambiguity turns up in your other example:
While you could interpret the second sentence to mean that the police blocked the road for only a short time during the week, most people would use these two sentences interchangeably. That's because of another quirk of the present perfect form: It usually emphasizes that the consequences of an action continue to the present even if the action itself has not.
To see why this matters, contrast the rain and blockade examples:
While both of these sentences are ambiguous, you can infer their meanings from context. In both cases, the perfect verb form indicates that the occurrence is still relevant in the present. For rain, it's easy to see how it's still relevant even if it has stopped falling: wet roads, muddy ground, dreary skies, and so on. In contrast, a blockade isn't very relevant unless it's still in place. Therefore, you can infer that the rain has probably stopped, whereas the blockade is probably still in effect.
Swan in Practical English Usage (p426) states:
You could say It has rained heavily since morning, but the progessive form places more emphasis on the continuing nature of the rain.
In the case of the road block, the simple perfect is used to indicate that the block was completed last Friday and its present result is that you cannot use the road.
The equally possible:
suggests more strongly than the simple form that the police have been continually present throughout the existence of the block.