There are very common mistakes when it comes to articles.

Mistake #1: "A" or "an" with plural countable nouns

You should not use "a" or "an" with a plural countable noun. Plurals are used when there is more than one. For example, "cats" means "more than one cat"; "people" means "more than one person". However, "a" and "an" means "one". If you say "a cats", there is a problem. Do you mean one cat, or many cats?

Some examples:

Mistake #2: "A" and "an" with uncountable nouns

You should not use "a" or "an" with uncountable nouns. Uncountable nouns are things that you cannot count. They are different shapes and sizes, like water, money or air. For uncountable things, "one" does not make sense. That means that (usually) you cannot say "a water", "a money" or "an air".

Some examples:

• I got water on my shirt √
  I got a water on my shirt X
• Do you have money? √
  Do you have a money? X

Mistake #3: Speaking in general

When you talk about things in general, you should not use "a" or "the".

If you are talking about countable things, use the plural. It is because in general, there are many of the things you are talking about:

• She likes cats (She likes all cats) General
  She likes a cat (There is one cat somewhere that she likes) Vague
  She likes the cat (She likes that special cat) Specific
• Horror movies are scary (All horror movies are scary) General
  A horror movie scared her (It is not important which horror movie it was, but anyway, she is scared) Vague
  The horror movie was cool (That horror movie that we saw together) Specific

If you are talking about uncountable things, you should use no article and no plural (because if something is uncountable, the plural is impossible):

• He likes money (He likes all money) General
  He likes a money Not possible
  He likes the money (He likes that money in particular) Specific
• Gasoline is expensive (All horror movies are scary) General
  A gasoline is expensive Not possible
  The gasoline is expensive (That gasoline at that gasoline station is expensive) Specific

Mistake #4: Using "a" or "the" with other determiners

A determiner is a word that points to a thing, like a, the, this, my, that, your, her, every, each and so on. Determiners say which one. You can only use one determiner. Some people use more than one determiner and it is strange. If you say "this my pen", is it "my pen" or "this pen"?

Some examples:

• I will get the car √
  I will get the my car X
• Have you seen this movie? √
  Have you seen this the movie? X

Mistake #5: Forgetting to say which one

Determiners are important for telling people which thing you are talking about. When you use a singular, countable noun, you need to say which one. If you tell me, "I like cat", I don't know what you mean exactly:

• Do you like all cats?
  I like cats
• Do you like a special cat?
  I like (the/this/that/my/our/your/every/each/either...etc) cat

Here are some examples:

• You can buy it at the store √
  You can buy it at store X
• Have you seen my bag? √
  Have you seen bag? X

When you tell me it is at the store, I already know which one you mean. If you ask, "Have you seen bag?" I don't know whose bag you are looking for. Are you looking for your bag, this bag, my bag, his bag, her bag, etc.?