There are no websites that can check grammar for you 100% reliably but students often find the Grammarly site useful (and no, I have no link with them so am not getting paid for mentioning them!). Online guides are best used as tools to remind you of the rules; they can’t be expected to be infallible.
The real answer of course lies in looking things up in grammar books. It’s not much fun but you’ll only get a reliable answer if you go to a good source.
Fortunately there are more enjoyable ways to improve your language skills. Reading literature in the language you’re studying will enable you to 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. Of course the reverse is also sadly true: if you read poorly written English all the time, your English language skills will suffer.
You could also try simply pasting a phrase into a search engine (use an English language one). 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. 🙂
For those of you who need more of a ‘quick fix’: the Daily Writing Tips site has useful advice on writing including grammar and I have posted advice on my Writing Skills page.
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).
Then write, read, write and read some more.