Usually the students are unaware of what they have done until it is pointed out.
Here are some common errors and some resources to help you overcome them.
Run on sentences.
Make sure you use full stops (periods if you are from the US). If your sentences have more than one main idea, then you should think about giving each idea its own sentence. Commas are not subsitutes for full stops (or periods). Learn to use both well. Click here to see how to construct your sentences to avoid this common error.
Consistent use of tense.
Sometimes, tense can move from present to past in student work and this diminishes the quality of the work.
The use of homonyms.
For example 'whether' or 'weather' Relying on a spellchecker just won't do!
Use of the article
Choosing whether to use 'a', 'an' 'the' can sometimes prove tricky, but luckily Purdue and UniSA have a few good resources such as Using articles: 'a' and 'the' or no article and A versus An or How to use articles (a/an/the)
Count or non count nouns
It's important to get these right as it also affects whether you use plural or singular verb form in your sentence and also the article. Look at this excellent explanation from Purdue. These excellent rules will help you decide if a noun is countable or not. However, the rules do not apply in all cases so if you are still unsure be certain to check a good dictionary.
The best way to improve your writing is to practise and practise some more!
All the best from the Grammar Gang!