"The cat has a tail."
Translation:לחתול יש זנב.
Hebrew doesn't like having two prefixes on a word, so when you would have two, they join together. You take the consonant sound from the ל prefix and the vowel sound from the ה prefix; you won't see the difference when written, but normally a ל prefix has more of a schwa vowel sound, whereas the ה prefix also gives you an "ah" vowel.
In Hebrew, instead of saying "he has", you would say, "to him there is". In all contexes.
I said, "יש לחתול זנב" and it was wrong, but I don't understand why it's wrong.
Your sentence is correct, it just emphasises that the cat DOES have a tail. The sentence duo gives says that the cat happens to have a tail.
I don't understand what he wrote, I think it's linguistic instead of layman but the ל ה issue is that if you use a prefix looks lamed, the hey is implied and not needed.
I think he means ל + ה, so you drop the hey it's implied with lamed, aka "to the"