Because enotes editors are only allowed to answer one question per post, I edited yours to the original question you asked.
In the slang expression, "don't have a cow," there are several things to notice. In terms of syntax, it is important to note that the expression is a command. Syntax refers to the principles that go into the construction of a sentence to create meaning. One common feature of command syntax is that the subject of the command can be omitted. In the case, the subject of the sentence (you) is left out but still understood.
Another feature of the expression to note is how the lexicon words chosen) express the semantics (meaning of the words chosen). In this expression, the speaker is using a lexicon of words to say "don't give birth to a cow." Everyone knows that a cow is a very large bovine animal, and everyone knows that humans cannot literally give birth to cows. That is were semantics comes into play. In this expression, the idiom or slang actually means "don't get so upset" or "don't go overboard". This is suggested by the idea of how completely over the top it would be for a human to give birth to a cow.
The English language is filled with these kinds of expressions, and an understanding of the true intent of the expression comes from a solid understanding of the English lexicon and the connotations and semantics that go along with it.