The sentence "Whether he was duplicitous regarding his character is open to question" is grammatically correct and complete.
What is your father is not a correct sentence. The word father is not a what but a who. The correct way to word it would be, who is your father?
This sentence is completely correct.
The correct word to use in that sentence is advised. Adviced is not defined as a word at all so it should not be used.
The correct verb in the sentence is 'has written'. The word 'written' is the main verb; the word 'has' is the auxiliary verb.
The correct grammar for the sentence is: "Please note that this letter is merely an inquiry regarding your interest and availability."
The sentence "Whether he was duplicitous regarding his character is open to question" is grammatically correct and complete.
What is your father is not a correct sentence. The word father is not a what but a who. The correct way to word it would be, who is your father?
My parents are very anxious regarding my study
The word meaning "concerning" is spelt as you have done in the question.
This sentence is completely correct.
The word 'springtime' is one word which should be capitalized as the first word in the sentence.The correct sentence is: Springtime has begun.
It means it must be grammatically correct. The word spellings and the structure should be correct too.
Should be added prediction regarding the joint investigation teams
no
The correct word to use in that sentence is advised. Adviced is not defined as a word at all so it should not be used.
I hope you really wanted the word "this" used in a sentence. This is a word that indicates an item or a location. This is the correct package. This is the correct street to turn right.