You use articles (a/an/the) before adjectives which are followed by the nouns in which they modify. If there is no associated noun, then they can't be used. Do not attach articles to adjectives in declarative statements which use adjectives following the noun. "The car" ---> "The fast car" "A dog" ---> "A happy dog" "My bag is yellow." ---> "My bag is yellow." (No article needed) As per your examples: 'She is young.' is correct. You can not use "She is a young." as a complete sentence. If you say it like that, people will ask you "She is a young what?" because they expect a noun to follow the adjective due to your use of an article. "This is a nice shirt." is correct. In this case the adjective 'nice' appears before the noun as a descriptive element rather than part of a declarative statement. So the article is needed.

Yes - BUT a noun must follow in the end. Also, it must be singular and non-specific; use 'the" to mark specific things. We use a/an/the to mark most things in a sentence. 1. "She is young", or "She is a young GIRL." 2. "This is a nice shirt". Skipping the "a" is not possible unless you use the plural and are talking generally: "These are nice shirts." Dropping the article a/an/the is only possible if you're talking generally, or giving the name of a person or place. More or less.