Yes, very!
The sentence "Is Mary not Happy" is correct grammatically as a question. It defines Mary as the subject and not happy as the object. In the other sentence, though grammatically correct, it could be confusing. Not Mary could be construed as all people but Mary. I would use the first sentence, as it is clearest.
Yes, the sentence "Mary is able to make a cake with assistance" is grammatically correct. It clearly conveys that Mary can make a cake, but she requires help to do so. The structure is straightforward, and the meaning is clear.
"More better" in any context is grammatically incorrect. The verb tenses are better and best. "Better than before" is grammatically correct. Example sentence: While the judges thought Mary's cake was better than before, they decided John's baking was better.
John and Mary, I am going to town, do you want to come too?
Mary and we. If Mary was not involved you wouldn't say "us" (i.e. "us like to go shopping").
John and Mary Everest
"My lawyer advised me to plead not guilty." "Mary will plead with her father and hope that he gives her the money."
"Here are John and Mary." 'Are' is used because "John and Mary" are plural.
The correct way to write the sentence is "It was Mary and Andrew." This is because "Mary and Andrew" is a plural subject, but the verb "was" agrees with the singular subject "It."
The sentence needs a subject to be correct."I want to present this award to Mary."
It depends on context and which kind of like is meant. Both of the following are correct:"Mary likes me" and "Why would she care for a man like me?"
John, where Mary had had 'had had', had had 'had'; 'had had' had had the better interpretation.