Here are a couple of mistakes that come up all too often. Maybe there are some people who might not be bothered by them, but I am sure there are lots of your readers and listeners who would be VERY bothered.
1. The use of ‘when’
Incorrect: ‘Scarlett O'Hara's dirtiest trick was when she lured Frank Kennedy into marrying her.’
Correct: ‘Scarlett O'Hara's dirtiest trick was luring Frank Kennedy into marrying her.’
Unless you are aiming to be a University English Grammar specialist, I suggest don’t worry about the reason. Just get used to using that construction and you’ll make life easier for your listeners and readers.
2. “The reason is because”
Do you say ‘The reason I don’t have my homework is because the dog ate it’?
If you do, please read on.
You see, even though lots of people say ‘the reason is because’ the fact is, it’s not correct English. And when you speak like that, and worse, when you write like that, you are making quite a public comment about . . . yourself!
Let’s look at the correct version: ‘The reason I don’t have my homework is that the dog ate it.’ Notice the difference? After ‘the reason’ avoid that word ‘because’.
Why is this so? Well, it’s a bit technical. Because an adverbial clause (a word or group of words answering the question "How?" When?" "Where?" Why?" or "To what degree?") cannot be used as a predicate nominative (a noun or pronoun or a group of words used as a noun or pronoun and answering the question "What?" or "Who?").
I suggest, forget about the ‘why.’ Just make sure you say and write ‘the reason . . . that’. Once you are used to it, it’s easy.
