Fumar es malo: where -ing is not the gerund

Trap 1: a subject or an activity is the infinitive

When English -ing names an activity, it is acting as a noun. Spanish uses the infinitive for that, never the gerund:

The test: if you could put "the" in front of the English word (the smoking, the cooking), it is a noun, so Spanish wants the infinitive. More on that form on the uses of the infinitive.

Trap 2: after a preposition, the infinitive

After any preposition, single words and longer ones like antes de alike, the verb is an infinitive:

English puts -ing after a preposition in all of these, which is why this is such a common gerund mistake. The old en + gerund (en llegando, as soon as he/she arrives) survives only in old-fashioned writing and some regional speech: do not copy it.

Trap 3: -ing describing a noun is que + verb

English lets -ing describe a noun directly: a box containing books. A Spanish gerund cannot, so rewrite with que and a conjugated verb:

Mistakes English speakers make

One trap catches good writers too: the gerund means "at the same time" or just before (at most an immediate result: se cayó, rompiéndose una pierna, he/she fell and broke a leg), not a later event. "He fled and was arrested later" needs y:

• Huyó y fue detenido después. - He/She fled and was arrested later. Not "huyó, siendo detenido después".