Here is how 'easy' it is to teach 'fronted adverbials' and 'subordinate clauses', according to the descriptions offered by the kind of 'grammar' that is being dished up for Spelling, Punctuation and Grammar tests. (Irony alert: it's not easy.)
1. In the sentences: 'When he went for a walk, he wore his hat.' And 'He wore his hat, when he went for a walk', the words 'when he went for a walk' in both sentences is what they call a 'subordinate clause', because it is supposedly 'subordinate' to the 'main clause', 'he wore his hat'.
(It is 'subordinate' according to the rules of this kind of grammar. In life, it might be just as important or more important that I'm telling you about the fact that he went for a walk!)
2. It is 'subordinate' because it's thought that it 'modifies' or 'qualifies' 'he wore his hat'. If you think that the meaning of him wearing his hat reflects on him going for a walk, you are wrong - according to this way of thinking! The 'subordinate' to 'main' mind-set views this as a one-way process only. All other ways are wrong.
3. Now we come to 'fronted adverbials'. Note: this is not in the same order of classification! It belongs to a different class of category. So, under this category, anything adverbial that comes before the 'main clause', is 'fronted' (i.e. 'at the front'). This could be a single word, like 'hurriedly'
or a phrase like 'In a hurry as usual'
or - wait for it - a 'subordinate clause' like 'as he was in a hurry'.
Hurriedly, he left cafe. [single word - fronted adverbial]
In a hurry as usual, he left the cafe. [phrase without a verb - fronted adverbial]
As he was in a hurry, he left the cafe. [phrase with a verb - sometimes called a clause, in this case a 'subordinate clause' - fronted adverbial]
4. In other words, dear teachers of 10/11 year olds, you have to tell your pupils that a 'subordinate clause' might be a 'fronted adverbial'. But also, it might not be. When they're out spotting fronted adverbials in their SPaG test, they mustn't go thinking that it'll be only one kind of 'construction' (i.e. sequence of words). It could be anything like any one of the three I've shown. The bit the 'believers' in this stuff are excited about, is that it comes BEFORE, the main clause. Whoooooo!
5. Why are they excited about it? Because, according to them, if you bung a few of them into a bit of writing, the writing gets 'better' or 'more interesting'. Why? I dunno.
6. You'll also remember that the 'as' in 'As he was in a hurry' is a 'subordinate conjunction' or a 'subordinating conjunction' - also something your pupils are supposed to spot.
7. So 'fronted adverbial' is a description of stuff you put before the main clause - of which there are several constructions possible. 'Subordinate clause' is a description of a clause that modifies the main clause or elements within a main clause. It has a 'header' word that this grammar calls a 'subordinate conjunction'.
8. Most of these descriptions are attempts at being purely and only about the structure of sentences and not about meaning or the function of such sentences in real life. It's like trying to describe any process - human body, how a car works, the way we lay the table - by trying to describe how one bit fits together, whilst not talking about what any of it is for or what kind of effect it's having.
I call this 'self-serving' or 'self-referential' and my own view is that it is of very limited use. I also think that it is a hoax that it is only supposedly about 'structure'. I think it's a hoax for the reason that you can only come up with these classifications and categories and terms, if you know what the meaning is in the first place. Yet, the terms themselves ignore the meaning and function! Even worse, we then set children tests in them which ignore even more of the meaning and function. It is what I would call 'abstract' or 'too abstract' or 'abstract to the point of being frequently useless and meaningless'.
It also leads to 'writing by numbers' - or extremely artificial writing techniques, that often end up as writing that sounds stilted and odd.