One of the most common complaints from students is that they don't get enough correction of their mistakes in class, perhaps because they think that is one thing they cannot do on their own. In the end, though, to speak or write good English you will need to be able to correct your own errors. This article has some suggestions on developing those self-correction skills and starting to use more accurate English, all without the help of a teacher.
One way to learn how to correct your own mistakes is to practice on other people's. If you Google "correct the mistakes", you will find lots of online exercises to practice this. Another useful source of exercises is the error correction task in FCE and CAE Use of English, in which you must cross out any words which are not needed.
You can combine correcting other people's errors with having your own corrected by getting together with other people to correct each others' work. As even native speakers find it difficult to edit their own work, you will find that you instantly agree with most of the corrections your partners make. If there are disagreements about which form is right, there are several ways of researching it:
Google the two possible forms and check which one is more common. For example, if you Google "the same than" and then "the same as", the latter has one thousand times as many results and so is obviously correct. If the result isn't so clear or you think it could be a common mistake even for native speakers, try the same thing with a Google Books search. It is important to always use quotations marks ("") around the search terms to make sure the words are together on the page.
Google it as a question. For example, searching for "Is staff countable or uncountable?" brings up lots of people trying to answer this.
Leave a question on an ESL forum, e.g. http://www.usingenglish.com/forum/
Leave a query on a site which specialises in answering questions, e.g. Yahoo Answers.
Look it up in a grammar book, e.g. by searching for it in the index
Ask a teacher, perhaps before or after the class if it is something that the other students wouldn't be interested in.
Most of those problems with unreliable error corrections would of course disappear if you had a native speaker to help you. Luckily, that is nowadays perfectly possible due to conversation exchanges, in which you teach someone your language and they teach you theirs. If you can't find someone locally, there are sites which specialise in setting up international conversation exchanges through Skype, Instant Messenger, etc. As the name suggests, most people mainly want to have the chance to chat in another language through conversation exchanges, and you might not make yourself popular if you expect your (unpaid) partner to correct pages of written work or correct every mistake you make. If it is okay with them, try picking out three or four parts of your written work that you are not sure about or one part of your spoken grammar that you want their feedback on.
If you are using a self-study book like English in Use, try doing the exercise before you read the grammar explanation. When you have read the explanation, see if there are any answers you would like to change. Then check your answers with the key at the back of the book, but don't mark or change any of the answers on the page. One or two weeks later, go back and look at your answers again and see whether you can remember which ones were wrong and why.
Another thing that you can do as you are doing grammar or vocabulary exercises and then checking your own answers is to write down the mistakes that you make in a separate notebook. Look through this once or twice a day, and tick off the errors that you are sure you won't make again. After a few weeks of this, you should have a list of spelling mistakes, grammar mistakes, etc that you often make. You can use this list after you have done a piece of written work, by looking through the work again to make sure that you haven't made any of the mistakes that are on the list.
When you have finished a piece of written work and gone through it looking for specific mistakes you often make, you can do the same again for more general typical problems like articles (a/ an/ the/ some/ any/ -), adverb word order, and prepositions (at/ on/ in/ etc). These can be checked against a page in your grammar book, with an internet search, or with a dictionary.
Try writing a piece of work with the automatic checking of spelling and grammar in your word processor program turned off. After checking your own work once, turn these functions on and see what things they correct, copying down any important ones into your common errors notebook. You could also print off the uncorrected version and the corrected version, and use that in the next few days or weeks to check your improved editing skills.
Even native speakers find it difficult to edit their own work, as your brain often sees what you were planning to write rather than what you actually wrote. A great tip for people who do need to edit themselves is to leave your work for at least two days and come back to it to do the final edit. You will be amazed how many mistakes you can now find. If you have spent those days doing any of the things mentioned above to improve your editing skills, you will also be able to pick out mistakes that you didn't know about when you originally wrote that piece.
You can also do the same thing with speaking by recording yourself and listening to it a day or two later. Mini-presentations where you speak uninterrupted for a minute or two are most useful for this, e.g. IELTS Speaking Part Two.