"to begin" is the infinitive. "began" is the simple past. "begun" is the part participle.

You use "begun" with the words "has" and "have" and their past tense counterpart.s Examples: I will begin my project soon. I have begun my project already. They began work on the building today. They have begun work on the building. It began raining this afternoon. It had begun raining by the time we arrived.

The verb "begin" which means to start is an irregular verb in English so it doesn't follow the rule of "ed". Therefore for present tense(infinitive): "Begin" I was about to begin my test. fore past tense: "Began" I remember I began to talk when I was just 2 years old. also for past participle: "Begun" The film has just begun.