That was a bad idea.

that Determiner

— Used to show which thing we are talking about; used with things that are not close to the speaker. Give me that book, not this one. Give me that, not this. That dog is hungry.

that Subordinator

— used to link a subordinate clause to a main one You said that you liked me. Give me the book that I dropped.

was Verb

— A form of the verb be used to talk about yourself, or another person in the past. I was singing in the shower. Who was that girl you were talking to?

a Determinative

— A is used when the following word could be any of a certain type. Compare "A book I saw on the shelf" and "The book I gave you yesterday".

a Noun

— A is the first letter of the alphabet. The letter "a" comes before "b". — In some schools, an A is a very high grade. Ron got an A on his earth science test.

bad Adjective

— If something is bad, it is not good. The man who killed his wife was very bad. Skipping school is a bad thing to do. Debts can go bad when a debtor has gone bankrupt. Could things get any worse than this? This is the worst time I've had all year!

idea Noun

— An idea is a concept or a thought. It was a good idea. — An idea is something abstract (not concrete). Love is just an idea. — An idea is a way to see something, a view. He changed her ideas on religion.