The most important differences are in spelling and vocabulary. Words ending with -or in American are generally ending with -our in British and Canadian English.
Examples: favor-favour, honor-honour
b) What is standard English?
Standard English is grammatically correct English. No slang words or expressions or syntax (gonna, wanna etc.), no contractions (don't, won't etc.)
c) What is a dialect?
A dialect is a variant of a language that uses different syntax or expressions and is generally pronounced differently. Dialects are variants of a language that are not the ''prestige dialect''. For English, the prestige dialect is either American English or British. Its dialects are Southern American English, African-American English, South African English and Australian English.
d) What are the characteristics of formal English as opposed to informal or spoken English?
Formal English as opposed to informal and spoken English is universally understood and agreed upon by all the English speakers around the world. It is the English set by conventions of standard English.
e) Do languages change over time?
Yes they do. Old English sounds nothing like Modern English. English has been influenced by Danish (during the Viking invasion), by Latin (at the time of christianization), by Norman French (Norman conquest of 1066) just to name a few. There was also the Great Vowel Shift which changed the pronounciation. Languages change over time for multiple reasons that are generally linked to the culture's ties with other cultures.