def correct_grammar(text):
    import re
    from string import punctuation
    
    contractions = {
        "cant": "can't",
        "wont": "won't",
        "dont": "don't",
        "doesnt": "doesn't",
        "didnt": "didn't",
        "isnt": "isn't",
        "arent": "aren't",
        "wasnt": "wasn't",
        "werent": "weren't",
        "havent": "haven't",
        "hasnt": "hasn't",
        "hadnt": "hadn't",
        "im": "I'm",
        "youre": "you're",
        "hes": "he's",
        "shes": "she's",
        "its": "it's",
        "theyre": "they're",
        "ive": "I've",
        "weve": "we've",
        "youve": "you've",
        "wouldve": "would've",
        "couldve": "could've",
        "shouldve": "should've",
        "thats": "that's",
        "theres": "there's",
        "heres": "here's",
        "whos": "who's",
        "whats": "what's",
        "wheres": "where's",
        "whens": "when's",
        "whys": "why's",
        "hows": "how's"
    }
    
    def fix_contractions(text):
        for wrong, correct in contractions.items():
            text = re.sub(r'\b' + wrong + r'\b', correct, text, flags=re.IGNORECASE)
        return text
    
    def fix_capitalization(text):
        sentences = re.split(r'(?<=[.!?])\s+', text.strip())
        sentences = [s[0].upper() + s[1:] if s else s for s in sentences]
        return ' '.join(sentences)
    
    def fix_punctuation_spacing(text):
        text = re.sub(r'\s+([,.!?;])', r'\1', text)
        text = re.sub(r'([,.!?;])\s*([,.!?;])', r'\1 \2', text)
        text = re.sub(r'\s*([\(])\s*', r' \1', text)
        text = re.sub(r'\s*([\)])', r'\1', text)
        return text
    
    def fix_possessives(text):
        text = re.sub(r'\b(\w+)s\b', r"\1's", text)
        text = re.sub(r"\b(\w+)'s\s+(own|house|car|book|dog|cat|child|children)\b", r"\1's \2", text)
        return text
    
    def fix_common_mistakes(text):
        common_errors = {
            "your": "you're",
            "there": "their",
            "then": "than",
            "loose": "lose",
            "effect": "affect",
            "to": "too",
            "alot": "a lot",
            "wierd": "weird",
            "definately": "definitely",
            "seperate": "separate"
        }
        for wrong, correct in common_errors.items():
            text = re.sub(r'\b' + wrong + r'\b', correct, text, flags=re.IGNORECASE)
        return text
    
    def ensure_sentence_ends(text):
        if text and text[-1] not in '.!?':
            text += '.'
        return text
    
    def remove_extra_spaces(text):
        text = re.sub(r'\s+', ' ', text)
        return text.strip()
    
    def fix_quotation_marks(text):
        text = re.sub(r'"([^"]*)"', r'"\1"', text)
        text = re.sub(r'\'([^\']*)\'', r'"\1"', text)
        return text
    
    def fix_subject_verb_agreement(text):
        text = re.sub(r'\b(he|she|it)\s+(go|run|jump|sing|dance)\b', r'\1 \2es', text, flags=re.IGNORECASE)
        text = re.sub(r'\b(I|you|we|they)\s+(goes|runs|jumps|sings|dances)\b', r'\1 \2', text, flags=re.IGNORECASE)
        return text
    
    if not text or not isinstance(text, str):
        return ""
    
    text = fix_contractions(text)
    text = fix_capitalization(text)
    text = fix_punctuation_spacing(text)
    text = fix_possessives(text)
    text = fix_common_mistakes(text)
    text = fix_quotation_marks(text)
    text = fix_subject_verb_agreement(text)
    text = ensure_sentence_ends(text)
    text = remove_extra_spaces(text)
    
    return text