How to show that a sentence is finished?

End marks are the punctuation you use to indicate that the sentence is finished. The end marks are the period (.), the question mark (?), and the exclamation mark (!).

Declarative sentences
Declarative sentences end in a period (.). A period indicates that the preceding sentence is a statement. It is the most common end mark for a sentence.

Example
I will pick Steve up at the train station tomorrow.

This sentence is a statement of fact, and is not particularly emotional; therefore, it ends in a period.

Interrogative sentences
Interrogative sentences end in a question mark (?).

A question mark indicates that the preceding sentence is a question.

Example
Will you pick Steve up at the train station tomorrow?

Exclamatory sentences
Exclamatory sentences end in an exclamation mark (!).

An exclamation mark indicates that the preceding sentence expresses a strong or sudden emotion.

Example
I forgot to pick Steve up at the train station!

This sentence conveys a greater sense of alarm than it would if you used a period:

I forgot to pick Steve up at the train station.

Note: Exclamatory sentences are rare, and one should avoid overusing the exclamation mark.

Example
I forgot to pick Steve up at the train station! He must be really mad by now! I don't know what to say to him! I don't want to tell him that I forgot!

Notice how the emotional impact of each sentence is actually blunted by the fact that all the sentences have exclamation marks.

Imperative sentences (commands)
Imperative sentences may end in a period or exclamation mark.

Examples
1. Get your feet off the couch!

2. Please take your feet off the couch.

Notice the difference in the emotional force of each sentence: the first is more urgent, though the second is quietly effective in a different way. Again, you should avoid overusing exclamation marks when writing imperative sentences.