Q:

How to use "to be" at the end of a sentence

I have some confusion about this; for example:

Where is the bank?
I don't know where the bank is.

I don't know how to be is used at the end of sentences.

A:

Let's start with a basic sentence:

The bank is there.

We can turn this into a wh-question by replacing there with where:

The bank is where?  (inappropriate in most circumstances)

This kind of question is acceptable in a few limited circumstances, but most wh-questions have two additional things done to them:

wh-fronting.  We move where to the front of the sentence:

Where the bank is ____?  (inappropriate in most circumstances)

It leaves behind a gap, which I've marked with a blank (____).  (The gap is usually not written.)  The word where and the gap refer to the same thing.  What's important is this: you can't add a complement after is because there's a gap there.
"Where the bank is?" is only appropriate in a few limited circumstances, like if you're repeating something back to someone.  It's usually incorrect.  To fix it, we need to do one more thing:
Subject-auxiliary inversion.  The subject (the bank) and the auxiliary verb (is) switch places:

Where is the bank ____?

Subject-auxiliary inversion marks a sentence as a direct question.  With both wh-fronting and subject-auxiliary inversion, we've created a complete question ("Where is the bank?").

However, if you want to use "where the bank is" as an indirect question, you don't switch the subject and auxiliary:

[ where the bank is ____ ]

This is a relative clause.  It doesn't work as a direct question, which means it doesn't work as a sentence on its own.  That's why I didn't capitalize where or put a period at the end.  But it does work as an indirect question, as part of a larger declarative sentence:

I don't know [ where the bank is ____ ].

In this example, we've fronted where, but we did not invert the subject and auxiliary.  That would be ungrammatical because it's not a direct question:

*I don't know [ where is the bank ____ ].  (ungrammatical)

(Technically, there are circumstances where it's possible to embed a direct question, particularly in speech.  But I'm ignoring that here.)

