"As... as" and "so... as" are CORRELATIVE CONJUCTIONS, and should be used properly in speaking and in writing.
"As... as" should be used in a positive situation.
Example:
Her voice is as powerful as other opera singers.
(positive situation)
"So..... as", on the other hand, should be used in a negative situation.
Example:
That bird is not so good as this bird.
(negative situation)
Wrong | She is not as tall as her mother. |
Right | She is not so tall as her mother. |
Wrong | His Kare-kare is not as delicious as mine. |
Right | His Kare-kare is not so delicious as mine. |
While using correlative conjunctions "as... as" one must be careful not to omit the second 'as'.
Example:
Wrong | This table is as good if not better than the old one. |
Right | This table is as good as, if not better, than the old one |
While using 'as', one should be careful as to avoid substituting it for 'that' or 'whether'.
Example:
Wrong | I am not sure as he is the actor we need. |
Right | I am not sure whether (or that) he is the actor we need. |