If you're learning English and are confused by some of the grammar, don't worry. Even native speakers often get the following rules wrong …
1. Capital letters
It's obvious that you need to use a capital at the beginning of a sentence, and we all know that place names and brand names also need one, but many people are confused about whether or not they should capitalise other words like titles. For example, is it 'the President' or 'the president'? Should it be 'president Obama' or 'President Obama'? The answer is that it should be 'President Obama', because it's referring to a specific person, but 'the president said', because it's not naming him.
2. Titles and headings
This confuses many students and anyone writing reports: should every word in a title or heading be capitalised? It's a matter of choice, but avoid capitalising conjunctions, prepositions and articles.
In recent years, BCE/CE have increasingly replaced the terms BC/AD. But whether you prefer the older terms or the new ones, you should get the position correct. AD precedes the year, whereas all other terms follow it. So you would refer to the death of Caesar happening in either 44 BC or 44 BCE, and the rebellion of the Iceni in AD 61 or 61 CE.
3. 'As' and 'since'
Even native speakers often use these two terms interchangeably, but they don't actually mean the same thing. They also make the mistake of using since to mean because. Since actually refers to time, thus 'I haven't seen him since university', but 'I was late for work because there was a train strike'.
4. -Ed or '-nt'?
This confuses a lot of people - how do you correctly use endings like 'learned' or 'learnt'? Are the two interchangeable? The rule is that learned is the past tense, thus 'I have learned everything', and learnt is the adjective, thus 'learnt behaviour' (not, as is commonly used, 'learned behaviour').
6. 'Like' and 'such as'
This is another example of words that are often used interchangeably, but shouldn't be. Which one you use depends on the meaning you wish to convey; both may be technically correct, but not actually say what you want to say. So if you say 'Reality TV like Big Brother is not worth watching', you are saying that any reality TV programme is not worth watching, whereas 'Reality TV such as Big Brother is not worth watching’, you are giving Big Brother as a specific example of a programme not worth watching. Confused? That's not surprising - the difference is so subtle that it's easy to miss.
7. 'That' and 'which'
These two words are often -mistakenly - used interchangeably. 'Which' refers to all objects, while 'that' specifies some of them. So you would say 'Should books, which are essential, be free?' if you meant that all books are essential, and 'Should books that are essential be free' if you were asking if those books that are essential should be free. Also remember that 'which' should be used in clauses, unless meaning 'which one'.
8. Word order
We know about the word order in simple subject-verb-object sentences, but what about longer sentences? Is there even a rule about which order they go in? Why does 'that old little charming lady' sound wrong, and 'that charming little old lady' sound right? It's because of a rule that nobody knows about. The order of adjectives should be opinion-size-age-shape-colour-origin-material-purpose-noun. So that she's charming is an opinion, then comes the fact of her small size, followed by her age.