a bolt from the blue

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.

bolt Noun

— A kind of thick screw that is used with a nut and sometimes a washer. — A a short heavy arrow used by a crossbow.

bolt Verb

— If you bolt you run away suddenly.

from Preposition

— When something is from someone, the person gave or sent it. I got a gift from my grandmother today. — When someone is from a place, that's where they started. I am Chinese. I come from Hunan province. Sorry I'm late. I just came from school. — You use from to talk about distances between thing in space or time. Saturn is far from earth. School is 5km from my home. The year 2515 is a long time from now.

the Determiner

— Used, instead of a, to reference something specific, already known to exist. Compare "I read a book." and "I read the book." — Used with a stress, to show that the word following is special. Are you the John Smith that I went to school with? — Used with an adjective that acts like a noun to mean all of the people concerned The poor are always with us. — Used with superlatives forms of adjectives and adverbs. You are the best.

blue Noun

— One of the colors people can see The sky was a deep blue above her and golden yellow beyond the village. The wall was a mix of watery blues and greens.

blue Adjective

— Colored blue. He had blue eyes. — If you are blue, you are sad. He was very blue that day. — If a time is blue, it is a sad time. It was a blue day at school when he failed the test.