I’ve just had an interesting query about websites to help with grammar. There are no websites that can check grammar for you reliably, although Microsoft insists that its software can help :-).
The real answer of course lies in studying grammar books and reading literature in the language you’re studying so that you internalise good writing practices without even realising it. Newspapers and most books are helpful here since someone will have proofread them and ideally removed errors in writing / typing before print. 🙂
For those of you who need more of a ‘quick fix’: http://www.dailywritingtips.com/ has useful advice on writing including grammar and I have posted advice here: http://www.english-lecturer.com/study-skills/writing-skills/.
You could also try simply pasting a phrase into Google: if you type in ‘he sitted down’ you won’t find anything because ‘he sitted’ is wrong; if you type in ‘he sat down’ and the phrase comes up on a reliable site then that’s a good indication that you’ve got it right. You might have to play around with vocabulary and limit your search to UK websites but this can be a surprisingly useful way to confirm that you’re doing something right – or warn you that you haven’t got it right quite yet. 🙂 Good luck.
PS Please don’t underestimate how good reading is for you :-). The best way to improve reading and writing skills is to read (ideally a good book or newspaper but even comics can help with learning a second language) and write and then do some more.