Life is strange sometimes. Some very strange things happen.

• Giải thích: "Life" is used here in a general sense to refer to the concept of life as a whole, not a specific life or instance.

I like skiing. But I’m not good at it.

• Giải thích: "Skiing" is used to refer to the activity or sport in general. When talking about enjoying or participating in an activity, the gerund form is used.

Who are the people in this photograph?

• Giải thích: "The people" refers to a specific group of individuals shown in the photograph.

What makes people violent? What causes aggression?

• Giải thích: "People" is used to refer to individuals in general, while "aggression" is an abstract noun referring to the behavior or attitude.

All the books on the top shelf belong to me.

• Giải thích: "The books" refers to a specific set of books that are on the top shelf. The definite article "the" specifies which books are being referred to.

The first world war lasted from 1914 until 1918.

• Giải thích: "The first world war" is a specific historical event, so it uses "the" to indicate that it is a particular war being referred to.

One of our biggest problems is unemployment.

• Giải thích: "Unemployment" is an abstract noun referring to the condition or state of not having a job. It does not need "the" because it refers to the concept in general rather than a specific instance