Present participle (verb-ing) clauses can be used without a subject pronoun and an auxiliary when the subject of the sentence does two things at the same time or when one action interrupts another.

It is really a matter of learning which verbs take 'to-infinitive' clauses and which take an '-ing' clause, Claudia. After some verbs, e.g. 'tell', 'advise', 'ask', 'to-infinitive' follows the ...