def verb_forms(s):
    """
    From a given verb makes 4-element tuple of:
        infinitive: The verb itself
                -s: The third form
              -ing: Continuous tense
               -ed: Past simple tense
        """
    words = s.split()
    verb = words.pop(0)
    third = cont = past = None

    if verb[-1] == '^':
        verb = verb[:-1]
        cont = past = verb + verb[-1]  # stop-s   # stop-P-ing # stop-P-ed
    elif verb[-1] == 'e':
        cont = past = verb[:-1]        # merge-s  # merg-ing   # merg-ed
    elif verb[-1] in 'sxz' or verb[-2:] in ('ch', 'sh'):
        third = verb + 'e'             # fix-e-s  # fix-ing    # fix-ed
    elif verb[-1] == 'y':
        third = verb[:-1] + 'ie'       # tr-ie-s  # try-ing    # tr-i-ed
        past = verb[:-1] + 'i'

    return tuple(' '.join([form] + words)
                 for form in ((verb),
                              (third or verb) + 's',
                              (cont or verb) + 'ing',
                              (past or verb) + 'ed'))