One thing you'll learn is that parrots decide what words they want to say, lol! And just because they don't say it doesn't mean they don't understand it. None of mine will say "bye bye." I don't have to wonder why; they don't like when I go bye bye. Rowdy says in this creaky, sad little voice, "Be right back?". Oobie whines like a dog. Merlin just looks sad, and Pete glares at me.
Merlin doesn't talk much. She prefers imitating sounds. But, she does everything I ask her to do, which amazes people.
Some parrots never talk, even if they are a species that is capable. Don't get a parrot for the hope it will talk. Choose the one that falls in love with you. They are finicky. They make instant decisions about who they like and don't like, and being nice to them won't necessarily win them over. Oobie absolutely does not like men or children. It's why I took her, since most households have one or both. They can also change allegiances. The person they loved as a baby may not be the person they love as an adult. Baby parrots see a chosen human as a parent. Adult parrots see a chosen person as their mate, and may try to kill everyone else in the household.
Cockatiels actually are cockatoos. They are just a smaller species in the Cockatoo genus.