thank you, Richard. I've corrected 'guys'- a silly typo =).
I know the difference between the Passive Voice and the Active Voice, this is not the point I am interested in.
How come that "a pool of melted water" is able to form ITSELF?? I'd rather say that it 'has BEEN formed". So, my question - is "has formed" the correct answer given in the Teacher's Book?
"has formed" in the active voice.
That means that the subject of the sentence does the action.
"A pool of melted water" is the subject of the sentence and it does something - it forms (in other words, the ice melts and becomes water and makes a pool of water.)
So we use the active voice and the present perfect tense to say that something started to happen in the past (= since 2002) and happens every year (continues to happen), hence the present perfect. TIP - the present perfect tense is often used when we see the time marker "since".
"has been formed" is the passive voice and that means that the subject of the sentence receives the action.
The passive voice requires an indication of who or what caused the pool of water to form, but there is no indication of that.
In the end, it is the subject that is doing the action, so the active voice, present perfect tense must be used.
Hope this helps...
By the way... You may want to edit your discussion. Change Gus (a man's name) to Guys :)
"a pool has formed or a pool of melted water has formed" is correct.
When ice melts, it forms/creates/makes/ a pool of water.
The pool is not forming itself. The melted ice (the water) becomes a pool of water if the surface is depressed enough to allow the water to come together in the form of a pool (rather than being on a flat surface where the water would form a thin layer of water on the surface below the water.
Is this explanation any clearer?
Other similar examples:
A storm has formed in the east.
A huge amount of snow has fallen.
The temperatures have risen lately.