'Who is talking' is usually a question, and would be written "Who is talking?"
However, if the person talking actually has the name 'Who', then "Who is talking." would be a correct English statement!
However, in view of the possible ambiguity, especially if no-one knows that the person talking actually has the name of 'Who', it might be better to rephrase the stament so that all readers/listeners will be in no doubt or confusion as to what you really mean.
One thing is very likely, Who is not asking this question!
It depends on what country you live in and what statement you are talking about.
No. The pronoun 'we' includes 'I/myself' so 'we' cannot be talking to 'myself'. These sentences are correct I am talking to myself. We are talking to ourselves.
It's correct English but not very good English
I do not think that the sentence is grammatically correct. The sentence seems ambiguous to me. Is the statement suggesting that "she has good command" of a group of people or animals? Or, is the statement suppose to imply that "she has command" of the English language?
"What I did is" is correct. It does not matter that the doing occurred in the past: it is still what I did, and will always be what I did. The answer below represents a very popular misuse of tenses. You are talking about something you did in the past, because you are using "did", therefore the correct statement is, "What I did was..." If you were to use the verb "is" or "to be" then that means you are speaking about the present, so the correct statement using this verb is "What I am doing is..."
'She does not have her book' is grammatically correct.
No, the correct statement is: That is mine.
The statement "I am happy for you" is correct whereas "I'm happy to you" is not in English.
Drop the 'ed' on 'longed' and it will be a proper statement.
Yes, it is correct to say "you are correct." It acknowledges that the other person's statement or opinion is accurate.
That is a correct English statement "I live in Houston." (the asker may wish to have this translated into another language, but did not specify which)
No, the correct statement is "Are you going to school?" using the verb "are" instead of "is."