You can use either.
Yes, that statement is correct as a way to indicate the passage of one month since the letter was posted.
No, "pre-planned since a month" is not grammatically correct. It would be more accurate to say "pre-planned for a month" or "planned in advance for a month."
"Former friend of Jane and me" is grammatically correct. In this case, "me" is the correct pronoun to use since it is the object of the preposition "of."
The correct phrase is "in est" in Latin. "Est in" would be grammatically incorrect.
Yes, it makes more since than my father and I. Either one is correct it depends on the situation. Your father and you can sit in the front seat. My father and I will sit in the back.
Yes, that statement is correct as a way to indicate the passage of one month since the letter was posted.
It is not correct to say "assist to". You would use this: I can assist you with your homework. The word assist would need to have a subject since it is a verb. The word 'to' is not a subject.
No, "pre-planned since a month" is not grammatically correct. It would be more accurate to say "pre-planned for a month" or "planned in advance for a month."
Yes :)
You can say it was a great game since the winning goal was scored by you. But most people say great game anyway when the match is over as you shake hands.
"I said to him" is a grammatically correct sentence fragment since "said" is still waiting for its direct object. You said what to him? "I said "to him". would be a grammatically correct sentence.
Fifty pairs. Since fifty is plural, you say pairs.
Logic tells me it is not correct since "suffice" is a synonym of sufficient. You would never say "sufficient it to say" would you. So why say "suffice it to say"?
"Former friend of Jane and me" is grammatically correct. In this case, "me" is the correct pronoun to use since it is the object of the preposition "of."
Since influence is not a physical thing with measurable dimensions, it would be better to say "great influence."
The correct phrase is "in est" in Latin. "Est in" would be grammatically incorrect.
Yes, it makes more since than my father and I. Either one is correct it depends on the situation. Your father and you can sit in the front seat. My father and I will sit in the back.