Some words belong to more than one part of speech. We could not come to know what part of speech a word is until we see what work it is doing in a sentence. A word can do different jobs in different sentences. Depending upon the function that the specific word does in the sentence, it takes a particular part of speech.
Look at the following sentences.
1. Give me some water.
In this sentence the word WATER is used as a noun to mean something.
2. They wafer the plants daily.
In this sentence the word expresses an action. It tells what they do. Here it is a verb.
Study the word fast in the following sentences.
1. He didn’t take anything during the fast. (It names something.) (The word FAST is a NOUN.)
2. Muslims fast during Ramzan. (It expresses an action and tells what Muslims do.) (The word FAST is a verb.)
3. I missed the fast train. (It adds to the meaning of the noun train. What kind of a train? A fast train.) (The word FAST is an adjective.)
4. She speaks fast. (It speaks about how she speaks.) (The word FAST is an adverb.)
Here are further examples.
We flew above the clouds. (Preposition)
Have you read the above sentence? (Adjective) See above: (Adverb)