Written language and conversational language are often
quite different. In fact, what's normal, common, and
acceptable in spoken language is often considered to be
unacceptable in written language. This is one of several
Hints on some of the differences.

In written language, reactions to what someone has said
are usually complete sentences, but in conversation, these
reactions are often quite abbreviated--and frequently only
one or two words. Here a few common examples:

Oh?

This one-word reaction shows that the listener is surprised
at what the speaker said--and it usually indicates that the
listener would like more information. "Oh?" is said with
question (rising) intonation.

A: Joe was in an accident.
B: Oh?

(B is surprised to hear that Joe was in an
accident. B probably also wants to hear
more details about the accident.)

Really?

This one-word question also shows that the listener is
surprised at what the speaker said--and, like "Oh?," it
usually indicates that the listener is interested in hearing
more details. It is also said with question intonation.

A: Joe was in an accident.
B: Really?

(B is surprised to hear that Joe was in an
accident and probably also wants to hear
more details about it.)

So?

This one-word reaction shows quite a different meaning:
that the listener thinks that what the speaker said is not
very important. It is also said with question intonation.

A: I got a really good grade on the test!
B: So?

(B doesn't care about A's grade on the test.
The grade is important to A, but not to B.)

So what?

"So what?" is like "So?" but stronger. When a listener
says "So what?," the listener is suggesting that what the
speaker said is not important at all--and that the listener
doesn't want to hear any more about it. When someone
says "So what?," the meaning is something like "Why is
this so important to you? I couldn't care less about it!"

Note: "So what?" is said with falling intonation.

A: I got a really good grade on the test!
B: So what?

(B cares nothing about A's grade and B also
doesn't want to hear any more about it.)

Hmm.

The sound "Hmm" shows that the listener is thinking
about what the speaker has said. It might be "translated"
as "That's interesting." Often "Hmm" is just a "polite
noise": the listener isn't really interested in hearing more,
but he or she doesn't want to seem rude

Note: "Hmm" is said with falling intonation.

A: I got a really good grade on the test!
B: Hmm.

(B isn't particularly interested in A's grade,
but doesn't want to seem impolite. B could
also have said "That's interesting.")

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Special Note:

"Oh?" and "Really?" are frequently combined:

A: Joe was in an accident.
B: Oh, really?

(B is surprised to hear that Joe was in an
accident and probably also wants to hear
more details.)