What is the difference between the terms adverb and Adverbial? The two are often confused and this isn't surprising, given that they sound so similar. Here are the National Curriculum definitions:


To my mind what the National Curriculum doesn't make sufficiently clear is that adverb is a grammatical form label (more specifically, a word class label), whereas Adverbial is a grammatical function label (and hence spelled with a capital letter on this blog). See my earlier blog posts about form and function (1) and form and function (2). Remember that adverbs typically end in -ly and often indicate the 'manner' in which something is done, e.g. quickly, happily, ruthlessly, slowly, reluctantly, etc. However, not all adverbs end in -ly: we also have very, often, always, sometimes, seldom, maybe, perhaps, and so on. And apart from 'manner' they can express a huge range of further different meanings, e.g. 'location', 'time', 'reason'. It's worth bearing in mind that we also have words like fast which are adverbs in some contexts, but adjectives in other. In the first sentence below it is an adjective that modifies a noun, but in the second it is an adverb that modifies a verb:
She loves to travel on fast trains.
The new high speed trains travel fast.
Adverbs can form adverb phrases. This is when we have two or more words, the most important of which is an adverb.* Very fast is an example. Here are some more examples:
We have always lived together [extremely happily].
You will [quite possibly] have to leave early.
She [nearly always] cooks chicken on a Sunday.
In these three examples the adverb phrases perform the grammatical function of Adverbial, but it is important to be aware of the fact that this function can also be performed by other kinds of units. The NC Glossary gives examples of three, namely a preposition phrase, a noun phrase and a subordinate clauses:

You also need to be aware of the fact that the same phrase can perform different grammatical functions, depending on its position in a sentence. Compare the following:
Last week was great.
I really loved last week.
We had a great time last week.
In each of these sentences from the point of view of form last week is a noun phrase, but the function of this phrase differs in each case: in the first sentence it functions as Subject, in the second it is an Object, whereas in the third example it is an Adverbial. What is a fronted Adverbial? This is simply an adverbial that is placed at the start of a sentence, as in the following examples:
Over the last few weeks, the train company has apologised several times for the delays. [preposition phrase functioning as fronted adverbial]
Last month, we went to the beach. [noun phrase functioning as fronted adverbial]
Before the match finished, the stadium emptied. [subordinate clause functioning as fronted adverbial]
(Something to be aware of: the NC insists that children write a comma after a fronted Adverbial.) The italicised phrases would normally occur later in the sentence, so a question that arises here is this: 'Why would we want to put Adverbials at the start of a sentence?' The answer is that fronted Adverbials highlight the phrases that have been placed initially, and hence they can be a useful device for writers to draw their readers' attention to this part of the sentence. For example, in the sentences above, the fronted Adverbials in each case signal that the 'time when' things happened is somehow important. Fronted Adverbials also offer writers the opportunity to vary their sentence structures, and for this reason teachers often encourage their students to use them. However, as with other stylistic features of writing there's a danger that the use of fronted Adverbials becomes a 'box ticking' exercise. So teachers may, for example, want to avoid setting a writing task that asks students to write a story with five fronted adverbials in it, without explaining why fronted adverbials are useful. To finish this post, two further points. You may be wondering whether last week in the sentence Last week was great is a fronted Adverbial. The answer is 'no': this phrase does not function as an Adverbial, and it's not fronted either. As we have already seen, this noun phrase functions as Subject here, and for that reason cannot have the function Adverbial as well, because any unit of language can have only one grammatical function. It is placed in the expected position for Subjects, and hence is not fronted. Finally, a word of warning regarding the term adverbial phrase. It's best to avoid using this label, because it confuses the function label Adverbial with the form label phrase. *Footnote: in the National Curriculum a word on its own is not regarded as a phrase, so that in the sentence She will arrive soon, the adverb soon functions as Adverbial, but is just an adverb, not an adverb phrase. (This is despite the fact that most linguists would say that words on their own should also be regarded as phrases.)