@ Dave - "Since 9am, I have been studying for the GMAT." OR "Since 11 am, I've been exhausted" -- both mark the beginning of an action (in the PAST), which continues into the present. Am I correct ?
Exactly right.
As far as TIME related usage of SINCE is concerned on the GMAT, SINCE is used with PRESENT PERFECT MOSTLY, I think. Right ?
If so, then could you please let me know EXACTLY where this PRESENT PERFECT is used ? I mean, is the PRESENT PERFECT used in the clause following SINCE or in the clause preceding SINCE in a SC in which SINCE indicates TIME aspect ?
Either way is acceptable. A simple example:
"I have been studying since 9am."
"Since 9am I have been studying."
Some OA's:
Part of the proposed increase in state education spending is due to higher enrollment: the number of students in public schools has grown steadily
since the mid-1980's and, at nearly 47 million, has reached a record high.
After analyzing data gathered by weather satellites, scientists report that the Earth's northern latitudes have become about ten percent greener
since 1980, due to more vigorous plant growth associated with warmer temperatures and higher levels of atmospheric carbon dioxide.
Since 1990 the global economy has grown more than it did during the 10,000 years from the beginning of agriculture