One of the most common mistakes in Spanish translations is the incorrect use of the gerund. A lot has already been said about the theory (and there’s plenty of information available everywhere), so I’m going to focus on the practical side: 3 simple rules to decide if the gerund in Spanish is used correctly or not.
Rule 1. The subject of the gerund must be the subject of the main clause.
Incorrect: Los bomberos entraron al edificio ardiendo en llamas. (Ouch!)
Correct: Los bomberos entraron al edificio que ardía en llamas.
Rule 2. The action of the gerund must occurr at the same time or before the action expressed by the main clause. Never after.
Incorrect: Los bomberos apagaron el fuego, apareciendo en las primeras páginas de todos los diarios. (Those must be the fastest newspapers ever)
Correct: Los bomberos apagaron el fuego y aparecieron en las primeras páginas de todos los diarios.
Rule 3. The action expressed by the gerund must have an adverbial function (when, how, why, etc.)
Correct: Los bomberos controlaron el incendio sacrificando sus vidas. (How)
The Spanish gerund never, ever, expresses a result (rule 2), but it can often express the cause. The above are very common uses of the Spanish gerund, but they’re not the only ones. Probably the most common mistake is to use the Spanish gerund at the beggining of the sentence and/or as the main clause:
Correct: Para empezar
Or better yet: Introducción
The Spanish Gerund can be very tricky sometimes. In case of doubt, don’t use it or ask somebody else. Just don’t discard it altogether. I’m writing this post because a proofreader removed all the Spanish gerunds in my translation, even though they all were used correctly. (Oh, and the text was about firefighters.)