A subordinate clause introduced by a modal conjunction describes the circumstances in which the action of the main clause is performed. It seems that modal conjunctions are simply inexistent in English and that modal clauses are in most cases translated in English with a preposition + gerund. (If one googles for modal clause / modal sentence one gets a lot of sites where the modal clauses in german are described, but none that describes its use in English.) However we get the idea by looking at the following example.
By telling the truth, we stopped the incipient rumor from spreading.
In order to convert this sentence into a modal clause, we need a conjunction that introduces a subordinate sentence with a verb.
~ By tell them the truth, we stopped the incipient rumor from spreading.
However this is not possible. By is a preposition, can be used together with a noun, for instance by chance, by the way or something that can have the syntactical value of a noun, for instance a pronoun, by him, or a verbal noun, by having this done you will learn.... The gerund is a verbal noun and should not be confused with the present participle. (Both are -ing forms, but their syntactical functions are completely different.) From that we can deduce that in English there is preference for shortening modal clauses with preposition + gerund and that is actually true. Anybody who has English as a foreign languages wonders about the frequency the gerund is used in English. We will see therefore in the next chapters a lot of examples, where we have a conjunction in Portuguese and the subordinate clause is constructed with a conjunction + gerundio and the English translation constructs with preposition + gerund. (Once again: The Portuguese gerundio corresponds to the English present participle and not the the English gerund.)