If your first language does not use articles ("a" and "the" in English), you may have trouble deciding which to use and when. I've tried to set down some rules here; while they should eliminate most common errors, they are probably not exhaustive.
There are four main issues to consider:
The usual answer is "yes". Two frequent exceptions are:
I saw you the Sunday before last.
The Guildford in Surrey is the prettiest of all the Guildfords in the world.
If you want to refer to a specific thing, especially a thing that has already been referred to, use the definite article "the":
Use "a" if you are not referring to a specific object:
Consider this example:
I will add a summary to my report. The summary should not be too long.
The idea of a particular summary does not exist prior to the first sentence, so we use "a". In the second sentence, there is now a specific summary that we can refer to (because it was introduced by the first sentence), so we use "the".
N.B. Use "an" instead of "a" before a vowel. (There are odd exceptions such as
The nouns we have considered so far are all countable. Some nouns ("sand", "milk") are not, so you can't really say whether they are singular or plural. For the purpose of deciding which article to use, these non-countable nouns behave exactly as plural, countable nouns.
This is only an issue for the indefinite article. For the definite article, use "the" for both singular and plural.
The plural indefinite article is either "some", or it is left out altogether. This is a slightly trick issue, but in general it is omitted when referring to a category:
Trees look at their best in the spring
as opposed to
There are some trees between those houses
Note that a singular, countable noun that is not a "proper" noun in the above sense must take an article.