Most rules of the English language have some irregularity [citation needed].
Forming the present participle isn't one of them: take the infinitive, and add the suffix -ing (in speech. In writing, it's a bit more complicated due to the irregularities of English orthography in general - but those irregularities are themselves regular, if that makes sense).
Why is this? Even "to be", the most irregular verb in English (with a whopping eight distinct forms, almost every one irregular), has a regular present participle.