Hi, I'm Thai, I appreciate your concern, however, if students can't even make a proper sentence, it would be very hard for foreigners to help. Most institutions here only want money, they don't really care if students make a progress or not.

Students also have a responsibility to study by themselves. If they really tried hard, they should be able to get better, with your advices of course. If they only come to learn from you, then back home with no homework or real determination to try hard, it is pointless to teach. They have been in English program for four years now but still did poorly, that means they have no real determination to get better. Why care?

In sum, I think the best decision is to keep your mouth shut. However, if my assumption is wrong, if these students are really want to improve themselves. My suggestion is to teach them basic grammar and suggest them to buy and read some basic vocabulary books. This is how non native speakers start from beginning. That's all you can do I think.