"There are five days left until the end of the month." We have to use "before" instead of "until" here.

Tom poured some milk into cat’s bowl. You need to have an article or other determiner before 'cat's bowl', for example 'the cat's bowl', 'his cat's bowl', etc? At what age did you get married? This is OK, but sounds a little awkward. 'How old were you when ...?' would be more natural. People envy you because you have what they don’t to. If you delete the 'to' at the end, this will be fine. There are five days left until the end of the month. Yes, this is fine. Have you ever been in love with someone? Yes, this is fine. I quarrelled with all my neighbours. This is fine, too. An American English user might remove the second 'l' from 'quarrelled' and the 'u' from 'neighbours', though.

All correct except for "People envy you because you have what they don't to." Remove the "to" at the end of the sentence - you can leave it as "because you have what they don't" or you can also say "because you have what they don't HAVE"