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Inversion: when two expressions switch places in the sentence and thus the ‘normal’ word order changes

Inversion here means  question word order   or   verb + subject word order   after certain expressions in the sentence.

So when does it happen? These are the cases:

I. After negative and restrictive expressions: the question word order is used

1. When there is a negative adverb or adverbial expression at the beginning of the sentence for emphasis, it is usually followed by the question word order, i.e. inversion. Such expressions include: at no time, not until …, under no circumstances, not only, only then, only after …
2. Inversion is also used after restrictive words placed at the beginning of the sentence such as rarely, seldom, hardly, little, never

e.g.#1: Under no circumstances are you allowed to enter the room.

e.g.#2: Not until it was too late did I realise how unhappy she was.

e.g.#3: Little did he know what was unfolding under his nose.

e.g.#4: Never have I been so happy.

II. After adverbial expressions of place: whole verb + subject

1. Common in spoken English after here, there and other short adverbs: Here comes the sun. (The Beatles)

After an adverbial expression of place or direction at the beginning of the sentence, intransitive verbs frequently come before their subject.

We find this mostly in literary style: On the grass sat a huge lizard.