It is rude to say this so please don't. It's here to practice vocabulary and structure.
Are you old?
Are we old?
Study English in London
There are some rules to help, but not as many as you'd like.
Negative prefixes can be added to words to change them from positive words to negative ones:
happy = positive
unhappy = negative (meaning not happy)
perfect = positive
imperfect = negative (meaning not perfect)
There are lots of negative prefixes. Let's look each in turn and any helpful rules:
Use with words beginning with 'r'
regular - irregular
rational - irrational
resistible - irresistible
Use with words beginning with 'l'
logical - illogical
legal - illegal
legible - illegible
Use with words beginning with 'm' or 'p'
possible - impossible
moral - immoral
passable - impassable
Use with words beginning with vowels (except 'i' and 'u') and consonants
accurate - inaccurate
eligible - ineligible
organic - inorganic
sane - insane
decent - indecent
The rest of the negative prefixes (un~, a~, dis~ and non~) do not have spelling rules. However, here are some helpful tips:
If the word is an adjective which comes from a verb then the prefix is probably (un~).
uninteresting (negative adjective)
uncomfortable (negative adjective)
If the original word is a noun it is probably 'non~'.
fiction - nonfiction
essential - nonessential
poisonous - nonpoisonous
De~ / Dis~
If you are making a verb negative then you would probably use 'de~' or 'dis~'
agree - disagree
honest - dishonest
trust - distrust
code - decode
Un~ or Dis~
Both come from verbs and so would be okay in most cases:
For example: agree
If we just want to make the verb negative, add 'dis' = 'disagree'
The adjective version of 'agree' is 'agreeable'. To make the adjective negative we add 'un' because the original word comes from a 'verb' = 'unagreeable'.
We could also make the negative verb (disagree) into an adjective = 'disagreeable'.
That means that both 'unagreeable' or 'disagreeable' are okay to use.
I'm sorry there aren't more rules to help. I'll continue to think of examples and add them to the answer as I go.