I assume that your grammar class will be conducted in the English language. In this case, mastering grammar in a second language is not easy to grasp and, as you mentioned, it can be a vague topic. I recommend making the class fun and engaging by incorporating humor. One way to do this is by using words that sound the same but are spelled differently in a sentence to illustrate the distinction, such as the words 'their' and 'there'. Similarly, you can also demonstrate the variance in the usage of punctuation marks, for instance, 'it's' and 'its'.
Here's a humorous example to highlight the importance of punctuation: An English professor wrote the phrase "A woman without her man is nothing" on the board and asked the students to punctuate it correctly. The outcome was quite amusing - all the male students punctuated it as "A woman, without her man, is nothing," while all the female students punctuated it as "A woman: without her, man is nothing."
Therefore, incorporating such examples and exercises can make your class more enjoyable and educational. You can easily find a plethora of examples on the internet to support your teaching.
I hope the above suggestions are helpful.
Regards, Harsh