An adverbial is a bit like an adverb, but it’s so much more than that. *swoon*

An adverbial can be an adverb, but it can also be any other kind of fragment of your sentence which describes how the verb happens. So, in a sentence like:

We slowly approached the castle gates.

‘Slowly’ is an adverb and also an adverbial because it describes how the ‘approaching’ action is carried out. On the other hand, in a sentence like:

I’m going into town with Chaz, alright Mum?

‘With Chaz’ is actually an adverbial here, because it gives more description to the action of ‘going into town’.

Some examples in German:

Mein Lehrer geht immer morgens um 11 Uhr aus, um eine Zigarette zu rauchen.

‘Immer’, ‘morgens’, and ‘um 11 Uhr’ are all adverbials because they all give us more descriptive information about the verb of going out to smoke a fag. We tend to call these time adverbials because…well, they talk about time.

Wir gratulieren dir vom ganzem Herzen zu deinem Geburtstag!

Here, the adverbial is ‘vom ganzen Herzen’ because it is describing how absolutely jazzed we all are to wish you a happy birthday.

Ich habe in der Dusche plötzlich bemerkt, dass meine Knie lila geworden sind.

‘In der Dusche’ and ‘plötzlich’ are both adverbials which give two different bits of information about how this person noticed that their knees had gone purple: ‘suddenly’ and ‘in the shower’. We have here an ‘adverbial of manner’ (‘plötzlich’, telling us the manner in which you do something, same as ‘angrily’ or ‘by bus’) and an ‘adverbial of place’ (telling us where you are: in the shower).