Hello Sawasdeekha

• These errors are common for Thai students. I think it's too late to teach them all the grammar rules, but pointing out sentence errors might be useful for them to self-correct their writing. There is an online grammar checking tool- you may want to tell your students about this
• http://www.gramlee.com/
• http://www.spellchecker.net/spellcheck/

Cheers kha