a and an are called indefinite articles. They are only used with singular nouns.
You must use some with plural or uncountable nouns. (e.g. friends, children, water, money)
a, an and some are used to show that the noun you are talking about is not known by you and the person you are talking to.
They are not used to show that we are talking about things in general. They are not used together with other determiners, e.g. my, this.
Some is most common in positive sentences. It can also be used in questions that encourage the answer 'Yes'. In other questions, any is normally used.
Using a or an depends on the first sound of the next word. You must be careful because the first letter and the first sound are sometimes different.
We use a when the next word starts with a consonant sound. e.g.
We use an when the next word starts with a vowel sound. e.g.
Write a, an or some for each of the given words.