To start with, here are some short rules. The point is that each of them illustrates the common error that it describes. Read them carefully, and be sure that you can see the error.
1. Verbs has to agree with their subjects.
2. Prepositions are not words to end sentences with.
3. And don't start a sentence with a conjunction.
4. It is wrong to ever split an infinitive.
5. Avoid cliches like the plague.
6. Also, always absolutely avoid and abjure annoying alliteration.
7. Be more or less specific.
8. Parenthetical remarks (however relevant) are (usually) inappropriate.
9. No sentence fragments.
10. One should never, ever generalise.
11. Contractions aren't necessary, and shouldn't be used.
12. Do not use no double negatives.
13. Eschew ampersands & abbreviations, etc.
14. Eliminate commas, that are, not necessary.
15. Never use a big word when a diminutive one would suffice.
16. Kill all exclamation marks!!!!!!
17. Use words correctly, irregardless of how others use them.
18. Use the apostrophe in it's proper place, and omit it when its not needed.
19. Puns are for children, not groan adults.
20. Proofread carefully to see if you any words out.
And here are a few more common trip-ups.
Which and That. There is no definitive ruling on this one, but the following examples exemplify a useful way of keeping these two words distinct:
"The dog that bit me was Kevin's". "The dog, which bit me, was Kevin's"
In the first case, "The dog that bit me..." defines the dog that we are talking about, so use that. In the second case, the fact that it bit me is adding information, not defining the subject, so use which and put the clause between commas.
They, this, that, these, those.... What a useful group of words, and how easy to make a pig's ear of them. Each of them refers to the subject of a previous sentence or clause, so make absolutely sure that the reader cannot mistake the noun to which they refer. For example:
"Type 3 axes have been studied by researchers at the University of Lowestoft. They have broad butts and are mainly found in the East of England." See?
Quite literally unique.... Three words that cause no end of trouble.
Quite is used differently in English English and American English. American English tends to use the old meaning of 'absolutely, completely', whereas English English tends to use quite as a sort of begrudging 'fairly, adequately but...'. So to be told that the meal you have just cooked someone was "Quite good" could be a real compliment or a bit of a back-hander. Use with care!
Literally means that the associated word or phrase is to be taken absolutely at face value, not as metaphor or euphemism. So "He hit the roof" may be a useful turn of phrase to describe a friend's anger on finding you dismantling his laptop with a can-opener, but "He literally hit the roof" would require that he actually came into percussive contact with the roof of the building. And anyone who has seen the 'Mr Creosote' sketch in Monty Python's The Meaning of Life would avoid ending a meal with the words "I am so full I could literally burst".
Unique means that there is only one of something, or that an event has happened for the first and only time. It does NOT mean 'fairly unusual', or 'I have never seen that happen before'. Therefore, please, try not to write "Stonehenge is almost unique". Either it is or it isn't.
The point of all of this is that English is a particularly rich and complex language, largely as a consequence of being a mongrel conglomeration of Germanic and Latin-derived languages. It is possible to say or to write something in English with great precision and clarity. That is why we make a fuss about bad English in essays. Poor use of language results in a lack of clarity, so that you may be saying something that you do not mean, or failing to express precisely what you do mean. Think about the meaning of the words that you use, and take care to say what you mean, and no more.
And finally, keep in mind the wise words of Danish physicist Niels Bohr:
"Never express yourself more clearly than you can think."