Can the word begun be used alone in a sentence?

No, "begun" cannot be used alone in a sentence. It is the past participle of the verb "begin," and therefore requires a helping verb to form a complete verb phrase.

Here are some examples:

Correct:

* We have begun our journey.

* The game has begun.

* They had begun their work early.

Incorrect:

* Begun. (This is a fragment, not a complete sentence.)

To use "begun" correctly, you need to include a helping verb like "have," "has," "had," "are," "is," "was," etc.