Why can't I use は with ある?


I wanted to say: there is a pen. Hence, I said

> ペンはある

But my girlfriend said to me that one has to say:

> ペンがある。

But she can't explain to me why, and I don't understand.

Do you have an explanation?  
Thanks in advance!




I feel that the distinctions between は and が are one of the hardest parts to
learn about the Japanese language, so I'll try to keep things mostly focused
on this example.

> ペンはある

Here "は” marks ”ペン" as a "topic", rather a "subject", but what that really
translates into is a feeling of:

> There _is_ a pen (but there isn't a ....)

This is because that "は" also is commonly used to indicate that what follows
it applies to the thing before the は, but _not_ to something else which may or
may not be directly stated.

Here is an example where the thing that doesn't apply is directly stated:

> ペンはあるけど、鉛筆はない

In fact は tends to be used more frequently with negative verbs (hence the は
part of ではない). So if you are saying "the pen isn't there", this would be
completely natural:

> ペンはない

However, if there was a situation where someone was panicking and somebody
asked them "What is wrong?" (どうしたの？), they could very well reply using が
instead:

> ペンがないよ！

This is because が strongly emphasizes the subject, and is generally used when
there was some uncertainty about a subject which is being clarified.

