Grammar
Grammar is one of those words that scares people, but it really needn’t. Every time you speak and write you use grammar. It’s just a system of language. You can study this system and learn its technical terminology, and this can be helpful in pinpointing and articulating to yourself what might be the issue with the piece of writing you have before you, but acquiring this knowledge all at once isn’t absolutely essential to improving others’ writing.
I’d recommend gradually building your knowledge of grammar, and you can start to do this with books like Rediscover Grammar by David Crystal or the Oxford A–Z of Grammar & Punctuation by John Seely, or by exploring Grammar Girl’s Quick and Dirty Tips online.
Remember that many authors won’t know all the technical terms of grammar either, and that your hanging a query on these terms could worry or even alienate them. It’s more helpful to become adept at explaining in simple language why a certain word or phrase sounds wrong.
If your author is using a dangling participle – for example in ‘Looking down the road, the cars were all red’ – you don’t need to mention that this is a dangling participle at all. You could address the issue with a query like: ‘This sounds as if the cars are looking down the road. You can either identify the person who was looking – “Looking down the road, Annie could see the cars were all red” – or rephrase to something simpler like “Along the road, the cars were all red”.’