Well, I'm assuming an "as" clause is one that begins with "As" in its meaning of "Whilst" or eg "As I was brushing my teeth, Periclaws the cat rubbed himself around my legs" ie it denotes two or more things happening at the same time. It has occurred to me since my first post above that a difficulty can arise if the two things are so unconnected they appear ridiculous eg "As I was brushing my teeth the wind whistled outside" -- there's no obvious connection apart from both happening at the same time, so it's a bit odd to link them with the "As" in that context, so I would red pencil something like that if I were an editor.
The present participle is (per Collins online) a participial form of verbs used adjectivally when the action it describes is contemporaneous with that of the main verb of a sentence and also used in the formation of certain compound tenses. In English this form ends in-ing
So "Walking down the street, I sang the National anthem" is a participial clause, because we have "Walking" and the action is happening at the same time as the main verb "sang". The -ing verb and the main verb must both have the same subject, so "Walking down the street, the wind blew my wig away" is wrong because the "walking" is something I am doing, so I am its subject, but the "blew" is something the wind is doing. So that sentence should be reworded eg into an "As" clause "As I was walking down the street, the wind blew my wig away".
And a clause is basically just a short sentence or a fragment of a sentence. (It's actually a lot more technical than that, but I don't understand the definition, so that's all you're getting from me!)
That help?
