Double-click on any word and see its definition from Cambridge Dictionaries Online.

"Is splitting of an infinitive considered bad English?"

http://www.wsu.edu/~brians/errors/nonerrors.html#split

"How do you change singular to plural?"

http://www.edufind.com/english/grammar/NOUNS2.cfm

"What are figures of speech?"

http://www.robibrad.demon.co.uk/Idioms.htm

"When do we use had better?"

http://www.better-english.com/grammarexplanations/hadbetter.htm

"What is the difference between slang and Cockney?"

According to the Cambridge Online Dictionary slang is:

informal language, which might include words and meanings which are not polite and which might stay in use only for a short time. It is used by particular groups of people who know each other, and is usually spoken rather than written:

			schoolboy slang
			army slang
			British slang
			a slang word
			a slang expression
			'Chicken' is slang for someone who isn't very brave.

Cockney is: (the way of speaking of) a person from East London, especially the poorer part:

			My father is a Cockney.
			You won't hear much real Cockney spoken unless you go to the East End of London.

and Cockney rhyming slang:

refers to slang which is used instead of a word or phrase and which rhymes with it.

In Cockney rhyming slang, you say 'apples and pears' to mean 'stairs'.

In short, Cockney (rhyming slang) is just one type of slang.

"What's the difference between pray and prey?"

http://www.wsu.edu/~brians/errors/pray.html

"What's the meaning of concordance?"

The Cambridge Online Dictionary says:

concordance noun [C] SPECIALIZED

a book or list which is an alphabetical collection of the words used in a writer's work with information about where the words can be found and in which sentences.

			a Shakespeare/Biblical concordance
			a concordance of magazine articles
			FORMAL a concordance to Keats

concordance verb [T] SPECIALIZED

We've got a computer program which will concordancenewspaper texts.