Learn English – the difference between “tedious” and “bored”? Are they identical or are there nuances

Either of these sentences may bear two meanings.

1. He purchased a book with the intention (which may or many not have been fulfilled at speech time) of giving it to me. Me is the Indirect Object of buy, exactly as it is in “He gave me a book” or “He gave a book to me”.

   This option between two methods of expressing the Indirect Object is called the ‘dative alternation’, and there has been a great deal of discussion among linguists about what determines which is employed. This paper gives a account of some factors which have been identified and a detailed statistical examination of their influence.

2. He purchased the book on my behalf—that is, he performed the purchase when it was impossible or inconvenient for me to do so.

Ordinarily He bought me a book will be understood to have meaning 1, while He bought a book for me may be understood in either sense, with the context determining which is more likely. For instance, if me was a child and he the child’s father, meaning 1 is more likely; but in a context like “James was very helpful; he bought a book for me while he was in London”, meaning 2 is more likely.

But these distinctions are not hard-and-fast; both constructions may be used in either sense.

There is a pretty visible distinction between both, and this example displays one of the main uses of continuous present.

You are being a jerk

Means that you're a jerk right now, on the matter at hand.

You are a jerk.

Is way more general and is used to express your opinion about the person. However, you could say "you're such a jerk" on the moment because you're furious, but then not think so any more.