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Hi All,
Why is "since past twenty years" wrong in the following sentence ? During past twenty year seems to be correct in the following
sentence..

1>
Since past twenty years, thousands of Indians have migrated to United States.
Can't we begin a sentence with a preposition- since ? Thanks a million..

2> Which one is better - since or because ?

Since past efforts to develop drugs that disrupt DNA-protein interactions have failed, Darnell believes that targeting protein-protein interactions is the next logical step.

Are there any rules to usage of 'since' to be begin a sentence with ?
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You can say "In the past twenty years, thousands of..." or

You can say "Since 1985, thousands of..."

Or "Since I graduated high school, thousands of..."

When referring to time, "since" needs to refer to a paricular time.

There was a time when purists frowned when people used "since" instead of "because," but it's very commonly used here (in the U.S.) now. I try to be careful of it in my writing, but in speech I know I use it all the time.
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A native speaker would say "For the past twenty years" or "Over the past twenty years" in that first sentence. The word since implies a point in time when the action started, whereas the words for or over indicate an ongoing event.

>>> Over the last decade the violent crime rate has steadily dropped.

>>> For the last twelve years I have lived in Houston.

In your second sentence, since and because would be interchangeable in informal conversation, but in a formal situation or in written work, the word because is a better choice.
We can use present perfect ( progressive / continuous ) tense as follow:

Present perfect ( progressive / continuous ) ..... since + point of time

1 I have known him since 1995.

2 I haven't seen her since Friday.

3 I have been living in London since I was born.

Present perfect ( progressive / continuous ) ..... for + duration of time

1 We haven't met Michael for two weeks.

2 She has been watching TV for three hours.

Since can be used as follow:

Result clause ... + since ( = because ) + reason clause ...

1 Michael didn't come to school since he was not very well.

2 Since she passed the entrance exam, she could join Medical College.