It is a word, an article to be exact. It is also a letter, you can only use it alone as a word.
"A" is a letter and word.
It is used to refer to something or someone specific.
It is also the first letter of the alphabet.
The word "a" is an Article of Speech, specifically an indefinite article.
The larva of a butterfly is called a caterpillar.
A sentence missing a subject or a verb is a fragment.
The word 'in' would be a preposition in the example sentence you gave.
It's the word that describes the noun in the sentence.
The last sentence.
A single word sentence is called a "monosyllabic" sentence in literature.
I called him on the telephone.
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A word. Many groups of letters is called a sentence(:
a sentence is called "une phrase" in French
My philosophy is called 'The Golden Rule'.
Your playmate just called.
You mean for the sentence you wrote? That's a question.
A translunar visitor is called an alien.
My supervisor is called Bob!
The security guard remained watchful throughout the night, ensuring the safety of the premises.
The recipe called for three sticks of butter.