As per English grammar, 'Since' is used to show the time period in the past from which the event is happening. So, is the sentence given below correct?
"Since more than six months, I have been directly assisting Steve as a Project Manager"
As per English grammar, 'Since' is used to show the time period in the past from which the event is happening. So, is the sentence given below correct?
"Since more than six months, I have been directly assisting Steve as a Project Manager"
It's not advisable to use 'since' like you have done, though it may still be correct.
You can use
I have been assisting Mr. XYZ for more than six months.
Or if you want to make a point that you have been assisting someone for a specific period of time, you can use
I have been assisting Mr. XYZ since January.
Assume January is six months before the current month.
Your "rule" is wrong. 'Since' is used to indicate a point in time past from which, not a period during which. 'For' is used to indicate a period.
I've only been married to Sally since July 1st.
It seems as though I've been married to her for æons.
The phrase 'as per' should be avoided. It strikes people as pompous and stilted. Say instead, "according to [the rules of] English grammar."
Don't capitalize 'project manager' unless it's an official title. (Speakers of British English may think differently about this, though).
Technically, according to your definition, this sentence would be (sort of) correct. However, the usage of the word 'since' in this case feels out of place and really should be written as:
"For more than six months, I have been directly assisting Steve as a Project Manager."