There is not a spell checker built into the website, but I use Firefox as my Browser and that underlines (in red) questionable spelling as I go along. Some I correct others I leave as it tries to change my UK English to American English !
As for the grammar, the best way to sort that, if it is a real problem for you, is to write the post in MS Word ( or some other program) which has built in checkers and then copy and paste onto here. If you do that you need to use the 'eraser' icon on the top left of each reply to take out any formatting and then it will be in the correct style and size of font.
However, on here the rules are not strict - although some would not agree with me. It is simply about writing in a way that is clear for the traumatized mind to read. Paragraphs broken down, using basic punctuation like full stops and capital letters.
I do find if I use a mobile device ( such as my phone) it is much more difficult and more errors slip through. But even then, you do have some time to make corrections after posting.
If you are really worried that you have made some terrible error and have run out of time to edit it yourself, you are welcome to 'report' your own post and request it be edited by moderators. You will see the 'report' button at the bottom of every post, and it is not just reserved for reporting spam etc.
Hope this helps?
Please don't worry about it!