def check_valid_input(letter_guessed, old_letters_guessed):
	"""
	the function will check if the user typed only one letter
	in English.
	False if:
	1. 2 letters or more
	2. if the "string" contains a symbol
	3. if the letter is already guessed

	:param letter_guessed: input from user
	:type letter_guessed: string
	:param old_letters_guessed: list of the letters already guessed
	:type old_letters_guessed: list
	:return valid_input: True or False acoording the following conditions
	:rtype: bool
	"""

	valid_input = True
	
	if len(letter_guessed) != 1:
		valid_input = False

	elif letter_guessed.isalpha() == False:
		valid_input = False

	elif letter_guessed in old_letters_guessed:
		valid_input = False

	return valid_input