Regular verbs in English consist of a base verb from which all different tenses can easily be formed. For example, the verb "to learn": present tense: learn; continuous present tense: learning, perfect tense and past tense: learned. As you see, all tenses of the regular verb "to learn" can be formed by adding the endings -ing and -ed.
English irregular verbs, however, have no definite rules, and although some irregular verbs have certain patterns in common, the best way to learn them is by looking at each individual verb. Let's look at the irregular verb "begin" in its simple present tense as an example.
Starting today, we begin again the work of remaking America.
Captions 26, 27: Barack Obama's Inauguration Day: Obama's Speech
As you see, in the simple present tense it remains the same. But in the present continuous tense:
It's beginning to turn into a lovely red color!
Caption 28: Tara´s recipes: Chilli Prawns and Golden Couscous
Just like a regular verb, this irregular verb adds -ing, but with an extra '"n": However, in the past tense:
She got a fright when the clock suddenly began to strike twelve.
Caption 15, Fairytales: Cinderella
The base verb "begin" changes to "began." And as a past participle:
Bottled water sales have begun to drop.
Caption 67, Nature Preservation: The Story of Bottled Water
The base verb "begin" changes to "begun."