Question

explain me basics of theory of automata?

Answer 1

That would take a significant amount of time. Go ahead and ask any specific questions about automata theory here if you'd like, however.

Answer 2

please tell me about language,string and word (theory of automata)
?

Answer 3

You have a bunch of different symbols that you call an alphabet. Think of letters and digits and punctuation and glyphs and doodles...
You can group these together into what are called strings or words. Think of "foo" or "Hf~r5|T" or "BEGIN" or "abbba" or "90210"...
Then when you make a list of words, you have a language. Think of a dictionary or a phone book or a grocery list...