The Verb "Do"

1)Why we add "do" with the main verb here,
I did send the massege.

2)What the difference between the sentence above and (I sent the massege).

Thanks in advance.

(First of all, it's "message" rather than "massege".)

The verb "do", in the example that you give, is being used as an
emphatic verb. Without "do", the sentence is merely a neutral statement of fact. With the verb "do", the sentence becomes more powerful and emphatic.

For example, you might use the emphatic version to contradict someone:

John: "You didn't send the message."
Jane: "I did send the message."

Or you might use the emphatic version if you're really happy and proud of yourself because you've sent the message:

John: "Did you send the message?"
Jane: "I did send the message."

Note, the use of "do" in English can seem a little strange. It is often optional, and its use, like in the above example, can add a different emphasis. But in other situations, a form of "do" is required -- for example to indicate the negative of an action ("I did not send the message", "he does not work there"), a form of "do" is required; there's no other good way to say it. (Well you could also say "didn't" or "doesn't".)

Sarah

Are you the same Sarah who posted a message in German on the language thread?

If so, Lazar is right, it is often used to contradict someone, and is comparable to the German use of 'doch'. For example:

John: Du hast mir die Message nicht geschickt
Jane: Ich habe sie dir doch geschickt (= I DID send it to you)