Tricky little things, -ing words. Writers often use them in the wrong places and for the wrong reasons, and they can turn serious situations into farce quite unintentionally.
The -ing ending occurs in different guises.
It’s part of a verb. That’s the present participle, used in continuous tenses both past and present (I’m loving it, I was enjoying it), and also frequently used as a sort of shorthand (The men watching the cricket cheered = The men who were watching the cricket cheered.)
There’s the gerund, where what is part of a verb plays at being a noun (the first sitting for lunch; my knitting is full of holes; bread and dripping).
If you use it as a verb, the –ing word has to have a subject. Use it as a noun, and it doesn’t – it is the subject.
You cannot have a present participle without a subject.
Running to catch the bus, the street shone with rain. Who is running? Not the street. There’s no subject here.
Shining in the puddles, Mark ran for the bus. Again, the subject – Mark – doesn’t belong to the participle. He’s not shining—he’s running.
Catching his breath, Mark ran for the bus. That’s better; Mark is the subject of all the actions. But see simultaneous actions below.
Reading is good for the brain as walking is good for the body.
It’s the equivalent of saying carrots are good for the eyesight. In the first part, reading and walking are gerunds, or verbal nouns. The line does not say that ‘you are reading’ or ‘you are walking’, but that the activities of reading and walking are beneficial. No specific person is implied – it’s a general statement.
Reading and walking are good for you. Carrots and broccoli are good for you.
But Reading and walking is not recommended.
Here reading and walking have both become verbs, and the combination of the two – that single is shows they have been bound together as simultaneous actions – alters the meaning. It’s the shorthand approach I mentioned earlier: Reading a book while you are walking is not recommended (which is not to say that it’s impossible.)
Some simultaneous actions should be used with care.
Walking across the road he ran up the steps. This is a physical impossibility. He can’t walk and run at the same time. There are (presumably) no steps in the road. Walking is a continuous state; he’s got to stop walking and start running up the steps. If you add while, you’ll see it more clearly: While walking across the road he ran up the steps. Not possible. He walked across the road and ran up the steps. These are two separate actions, and therefore acceptable.
Walking across the road he took out his phone. Yes, he can walk and put his hand in his pocket for his phone at the same time.
Continuous tenses need care too.
She watched them walking together, cracking jokes, smiling at their happiness. Who is doing the joke-cracking and smiling? It’s not clear. She could be smiling at their happiness; they could be cracking jokes. Maybe she does both. The continuous verbs need subjects – we need to know who owns which action. She watched them walking together and cracking jokes, smiling at their happiness. Better: we know that they are walking and cracking jokes, but the smiling is still slightly ambiguous. Better to change it to and smiled – we have her previous action described in the past tense, so this one will match it.
Cantering up the drive he swept her into his arms, closing the door with one booted foot. Multi-tasking heroes are one thing, but impersonating a horse is quite another. Make sure you aren’t making a laughing stock of the poor man.