$noun1 = '\w+\/NN ';     #a variable to define noun singular
$noun2 = '\w+\/NNP '; #a variable to define proper noun singular
$noun3 = '\w+\/NNS';      #a variable to define noun plural
$noun4 = '\w+\/NNPS'; #a variable to define proper noun plural

$verb1 = '\w+\/VB';     #a variable to define verb base
$verb2 = '\w+\/VBZ';  #a variable to define verb 3rd person singular present
$verb3 = '\w+\/VBP';  #a variable to define verb non 3rd person singular present
$verb4 = '\w+\/VBG';  #a variable to define verb, gerund or present participle
$verb5 = '\w+\/VBD';  #a variable to define verb past tense
$verb6 = '\w+\/VBN';  #a variable to define verb past participle

$prpplural = 'I|You|We|They|i|you|we|they\/PRP';
$prpsingular = 'He|She|It|he|she|it\/PRP';

print "Enter a file name:\n"; # get input from user for a filename
$namafail= <STDIN>;

$fail="$namafail";
open (in, $fail) || die "Cannot open $fail for read :$!\n";
$outfile="c:/out.txt";
open (OUT, ">$outfile") || die "Can't open $outfile for writing :$!\n";

print OUT "This is the output for file: $fail";

while ($line =<in>) {
@word = split/\n/, $line; # splits each word in the line including leading whitespace

   foreach $info (@word) {
   $firstindex = $word[0];
    
     if ($word[$firstindex]=~ /$noun1/){
     &check_presenttense_singularNN;
     }elsif ($word[$firstindex]=~ /$noun3/){
     &check_presenttense_pluralNNS;
     }elsif ($word[$firstindex]=~ /$noun2/){
     &check_presenttense_singularNNP;
     }elsif ($word[$firstindex]=~ /$prpplural/){
     &check_presenttense_pluralPRP;
     }elsif ($word[$firstindex]=~ /$prpsingular/){
     &check_presenttense_singularPRP;
}#end of 1st while

sub check_presenttense_singularNN{
        
if ($word[$firstindex]=~ /$noun1/){
     if ($word[$firstindex]=~ /$verb2/){
     printf OUT "\nThis sentence has no grammatical error: \n$word[$firstindex] \n";
     }
     else
    {
     printf OUT "\nThis sentence has grammatical error: \n$word[$info]";
        
     @sentence = split / /, $word[$firstindex];
       for $error (@sentence) {
            
    if ($error=~ /$verb1/|/$verb4/|/$verb5/|/$verb6/ )
    {
    printf OUT "\nError in tense in the above sentence is at:\n$error \n";
    }
       }#end of for
     }#end of else    
   }#end of if
}#end of sub

sub check_presenttense_singularNNP{
        
if ($word[$firstindex]=~ /$noun2/){
    if ($word[$firstindex]=~ /$verb2/|/$verb3/){
    printf OUT "\nThis sentence has no grammatical error: \n$word[$firstindex] \n";
    }
    else
    {
    printf OUT "\nThis sentence has grammatical error: \n$word[$firstindex]";
        
       @sentence = split / /, $word[$firstindex];
       for $error (@sentence) {
            
         if ($error=~ /$verb1/|/$verb4/|/$verb5/|/$verb6/ )
         {
         printf OUT "\nError in tense in the above sentence is at:\n$error \n";
         }
       }
     }#end of else    
   }#end of if
}#end of sub


sub check_presenttense_pluralNNS{

if ($word[$firstindex]=~ /$noun3/){
   if ($word[$firstindex]=~ /$verb2/){
   printf OUT "\nThis sentence has grammatical error: \n$word[$firstindex] \n";
        
     @sentence = split / /, $word[$firstindex];
       for $error (@sentence) {
            
         if ($error=~ /$verb2/|/$verb3/|/$verb4/|/$verb5/|/$verb6/ )
         {
         printf OUT "Error in tense in the above sentence is at:\n$error \n";
         }
       }      
    }
    else
   {
    printf OUT "\nThis sentence has no grammatical error: \n$word[$firstindex] \n";  
  }
}#end of sub

sub check_presenttense_pluralPRP{

if ($word[$firstindex]=~ /$prpplural/){
     if ($word[$firstindex]=~ /$verb2/){
     printf OUT "\nThis sentence has grammatical error: \n$word[$firstindex] \n";
        
       @sentence = split / /, $word[$firstindex];
       for $error (@sentence) {
            
         if ($error=~ /$verb2/|/$verb4/|/$verb5/|/$verb6/ )
         {
         printf OUT "Error in tense in the above sentence is at:\n$error \n";
          }  
       }      
     }
     else
    {
     printf OUT "\nThis sentence has no grammatical error: \n$word[$firstindex] \n";  
   }
}#end of sub

sub check_presenttense_singularPRP{

if ($word[$firstindex]=~ /$prpsingular/){
    if ($word[$firstindex]=~ /$verb1/){
    printf OUT "\nThis sentence has grammatical error: \n$word[$firstindex] \n";
    
      @sentence = split / /, $word[$firstindex];
         for $error (@sentence) {
            
            if ($error=~ /$verb1/|/$verb3/|/$verb4/|/$verb5/ )
            {
            printf OUT "Error in tense in the above sentence is at:\n$error \n";
           }
         }      
     }
     else
    {
    printf OUT "\nThis sentence has no grammatical error: \n$word[$firstindex] \n";  
    }
}#end of sub
}
}
}
}
}

if ($word[$firstindex]=~ /$singularplural/){

use strict;
use warnings;