**Traditional grammar:**

The rule usually trotted out by traditional grammars regarding where to put adverbs such as the word *finally*, is quite complicated, and goes something like this:

> 1. If there is no auxiliary verb, put the adverb before the main verb:

 - They finally arrived.

> 2. However, if the main verb is the verb BE, then put the adverb ***after*** the verb:

 - They were finally free.

> 3. If there is an auxiliary verb and a main verb, put the adverb between the auxiliary and the main verb.

 - They had finally succeded.

This can be quite tricky for students, as can might imagine. However, it gets more forbidable, because certainly in grammar books for EFL students, it is rarely stipulated what to do when there is more than one auxiliary verb.

**Modern grammar:**

In modern  grammars the rule can be stipulated much more simply. Modern grammars recognise auxiliary verbs as a class of verb which have similar syntactic properties, rather than verbs that come before a main verb. The verb to BE, therefore, is always an auxiliary verb, regardless of whether it is taking another verb (phrase) as its complement. 

We can then simply say that:

>  5. The adverb must be in a post-auxiliary position. 

This means we can put it after *any* auxiliary verb. Even when there is no auxiliary we can still identify the position that we would put an auxiliary verb if it was required:

 - In the end they [x] find their dinosaur.
 - In the end they [do] find their dinosaur.

In such sentences the adverb goes in the post-auxiliary position:

 - In the end they [finally] find their dinosaur.

From the simple rule stated in (5), we can correctly derive the following sentences where the relevant auxiliary position has been marked in bold:

 6. They **[x] finally** arrived..
 2. They **were finally** free.
 3. They **had finally** finished.
 4. They **will finally** delete them.
 5. They **will finally** be deleted.
 6. They will **be finally** deleted.
 10. They **will finally** have been deleted.
 11. They will **have finally** been deleted.
 12. They will have **been finally** deleted.