I am always quite surprised when even my good students make what I think are simple mistakes in their writing. One mistake which I see again and again is when the verb in the sentence doesn't agree with the subject.
Many immigrants who come to Britain works either in highly-skilled or unskilled jobs.
The subject of the verb, immigrants, is in the plural, so the verb works must be in the plural form too. The correct sentence would be:
Many immigrants who come to Britain work either in highly-skilled or unskilled jobs.
It is difficult to explain the effect that mistakes like this have on the reader. The reader often has to stop, think, try to understand, and then carry on - the whole reading experience is made harder and your mark will go down! When you have finished work you should edit it, that means checking it for mistakes. Don't just read it through, but check it for a particular type of mistake each time.
How to check for subject-verb agreement:
1. Take your first sentence.
2. Circle the subject. Think: is it singular or plural?
3. Circle the verb which goes with that subject. Does it agree? Correct it if you need to.
4. Do the same with the next sentence or clause.
This technique is also good for checking that you have got a subject and verb in your sentence. If you can't find the subject and verb then you need to think again about your sentence.
There is a good online exercise to help you raise your awareness of subject-verb agreement here. It is rather nicely designed with feedback provided. (Use the NEXT button at the top right to continue to the next question.) Finally, make sure you edit your work for subject-verb agreement each time you think you have finished your work!