Online English Grammar checker??? errrr
http://spellcheckplus.com/ MAYBE?
US and English are almost the same and identical grammatically for your purposes.
I think old MS word used to have a grammar checker also.
Most English speakers use awful grammar anyway but as we all use the same mistakes it just goes unnoticed.
You just need to learn to use the same mistakes we do and you'll do great
again, good luck.