A more adult view of the world
At around 19 months, your toddler will begin to understand when things don't conform to grown-up ideas. She'll laugh at something that's obviously wrong -- like calling a giraffe a zebra or calling a brother a sister. She'll also recognise when something isn't how it should be, like a stuffed bear with an eye missing or a new stain on the carpet.
Although you're probably still understanding less than half of what your toddler says, she's beginning to understand most of the words you use around her. She's also having a vocabulary spurt and putting together pairs of words, like "Me go" or "You put."
Bossiness and negotiation
To reduce power struggles as your toddler strives for independence, make sure you treat her the way you'd like to be treated. Try compromise instead of commands. Instead of "you can't go outside now," you might say, "we can read a book now and go outside later." Instead of "you must pick up the blocks before dinner," you could make it into a game: "I'll pick up the puzzle pieces while you pick up the blocks."