Use A or AN with singular nouns that you can normally count, like "woman, park."
She's an intelligent woman.
It's a beautiful park.
Use nothing with plural nouns, like "dogs, children," and nouns you normally cannot count ("milk, weather.")
They're beautiful dogs.
It's stormy weather.
The timer gives you 15 seconds to find the answer. It notes your errors but it only shows you the correct answers at the end.
If you don't use the timer you see the correct answers immediately.