Examples:

• He travels to work by train.

  ( Not : He travels to work by a / the train.)

• She loves taking photos of caterpillars.

  ( Not : She loves taking photos of the caterpillars.)

Examples:

• Cars are still expensive for lower-income people. (Countable noun)
• Flying is far safer than driving. (Uncountable noun)

• He loves drawing wild horses . (Countable noun)

  ( Not : He loves drawing the wild horses .)

• If we go by train, we will save time on our journey. (Uncountable noun)

  ( Not : If we go by train, we will save the time on our journey.)

Examples:

for general statements:

• Pigs are farm animals.

  ( Not : The pigs are farm animals.)

• Bats are nocturnal creatures.
• Ice floats on water.

before plural countable nouns:

• He bought red plastic roses for her.

  ( Not : He bought the red plastic roses for her.)

• We traveled through different countries, working in restaurants.
• She is inviting only female friends for her birthday dinner.

No article is required before uncountable nouns.

• We drink alcohol-free beer for dinner.

  ( Not : We drink the alcohol-free beer for dinner.)

• Those statues are carved in white marble.
• Trees absorb carbon dioxide which animals breathe out.

before abstract nouns used in a general sense:

• It is said that money cannot buy happiness.

  ( Not : Money cannot buy the happiness.)

• We always expect total honesty from our employees.

  ( Not : We always expect the total honesty from our employees.)

before proper nouns:  

• He is a fan of Michael Jackson.

  ( Not : He is a fan of the Michael Jackson.)

• She gave birth to twins in June.

  ( Not : She gave birth to twins in the June.)

before illness:  

• She came down with measles and had to stay in bed.

  ( Not : She came down with the measles and had to stay in bed.)

• Our family doctor suffers from asthma.

  ( Not : Our family doctor suffers from the asthma.)

before name of materials: 

• Gold is found in Australia.

  ( Not : The gold is found in Australia.

• They are mining for tin.

  ( Not : They are mining for the  tin.)

before the name of cities, states, countries, islands, or mountains: 

• India achieved independence in 1947.

  ( Not : The India achieved independence in 1947.)

• Mount Everest is the world's highest mountain.

  ( Not : The Mount Everest is the world's highest mountain.)