To make your search more powerful use the following tips...

AND
e.g. tax AND UK - brings back pages with the words 'tax' and 'UK' in them

OR
e.g. tax OR UK - brings back pages with 'tax' or 'UK' in them (possibly both)

NOT
Use this to filter out words that are not required
e.g. tax AND NOT UK - brings back pages with the word 'tax' but that do not contain 'uk' in them

Quotes may be used if a certain expression is required instead of single words
e.g. "income tax" AND NOT "CAPITAL GAINS" - brings back pages which contain 'income tax' but not 'capital gains'.

Note:
The search is not case sensitive, so searching for 'tax' or 'TaX' will bring back the same results.