[Inversion] Very slowly he understood it // Very slowly did he understand it.

[wotcha]

1. He understood it very slowly.

2. Very slowly he understood it.

3. Very slowly did he understand it.


My grammar book says if we place 'very slowly' at the front of the sentence for emphasis,

there occurs an inversion like sentence 3.


I wonder which is grammatical between 2 and 3.

[Rover_KE]

You need a better grammar book.

For a start you don't understand things slowly, so I'm not going to address those sentences.

Consider these instead:

1. He approached the growling dog very slowly.

2. Very slowly he approached the growling dog.

3. Very slowly did he approach the growling dog. Possible but not colloquial.

Rover