You chop up sentences into various categories. Here are some of them: nouns, verbs, adjectives, adverbs, articles, pronouns, prepositions, conjunctions. Right away there's a problem with several of these: where nouns have a clear function in terms of a structure of a sentence, adjectives are handmaidens to nouns, but adverbs are handmaidens to verbs, adjectives and whole sentences. They are of a different order of feature. Now we slot some of these into functions - subject, object and indirect object but 'verb' doesn't have a special name for its functional role. A verb is a verb. Now we stick these into some other structures - phrases and clauses. However, we can't agree what's a phrase or clause. Now we have the idea of a 'main clause' and a 'subordinate clause'. But the problem is that there are some main clauses e.g. ones beginning with 'but' that are really as subordinate as some subordinate clauses. And there are some subordinate clauses which are as important as some main ones. The term is dodgy. Now we have 'relative' clauses. These are a kind of subordinate clause. But they can also go into noun phrases...or noun clauses...if we knew exactly what the difference is between phrases and clauses...which we don't. No matter, this is all really easy. Now we have some descriptions of types of sentence: commands, exclamations, questions and statements. Easy, other than that we can ask questions by making statements, and we can make commands without using the 'command' structure of the verb 'the imperative'. And we can exclaim without the words 'how' and 'what' in whole sentences. But these don't count. Still with me? Now to fronted adverbials. Fronted adverbials don't have to be adverbial. They are basically any old phrase that appears in front of a main clause. But that doesn't include what's in the noun phrase. It has to come before the noun phrase at the head of the main clause - except when that main clause is a question which in English involves inversions. So a fronted adverbial can include elements which can also be in other categories like for example a relative clause, a subordinate clause (bearing in mind all relative clauses are subordinate clauses but not all subordinate clauses are relative clauses) but can also include adverbial phrases (bearing in mind we don't really know the difference between phrases and clauses) and adjectival phrases, and while we're on it these involve determiners which includes a whole set of features which appear before nouns all of which must be determiners and nothing else, though I should point out that some of these features can be a single word or several as with 'all of' or 'each of' but when we say 'every day' though it looks like an adjective, it isn't an adjective because it's a determiner, though why it's a determiner and not an adjective isn't clear even though an adverb can modify it as in 'nearly every day' so after all that adverbs can also modify determiners. Got it?