None is more important! The more you advance in learning you will be prone to use both adverbial and attributive clauses to enrich your style and form more intricate sentences. Both are important for the understanding of written text and the ability to write skillfully attaining a certain level of accuracy in meaning and a refinement of one's turn of phrase.

Try and remember all the English words and phrases you know. Go on youtube and look for TV shows that are in English but with subtitles. It may take a while, but you'll start to recognize words, or work some out. Also, find kid's books in English and look through them.

in some way, an 'attributive' clause tend to misleads the readers focus on the main idea (i.e. about the character or about what was spoken). ex: "please leave me alone" cried the seemingly afraid Edward, who is sitting in front of his sick father while looking afar from his window. however, as far as learning english, I think this would also be useful (just don't abuse it). :)

resite good english articles is very important.

Just Practice, read daily news paper .