Hi,
1) I teach English to the 10th class. OR I teach English to a 10th class. ??
If there is only one "10th class", use 'the'. If there are more than one 10th classes, use 'a'.
When I went to school, it was called 'grade 10', and I think it still is.
If there is only one grade-10 class in that school, say 'the'. If there are more than one grade-10 classes, say 'I teach English to 'a grade-10 class' or '.....to one of the grade-10 classes.'
2) I heard about it through a friend. AND I heard about it from a friend. What's the difference?
I think, to many people, both sentences mean the same thing, that a friend told you about it.
But if a friend directly told you about it, it seems better to use "from a friend".
"through a friend" at least raises the possibility that you didn't hear about it directly from your friend. Maybe your friend knew about it and then (s)he told another person and that other person told you about it. The messaage passed through your friend on its way to you.
Some people may say that even if your friend told you directly, the message still passed through your friend. And they could be right in saying that. But it is also possible for the message to have passed through your friend and through another person before reaching you. In that case you didn't hear it from your friend.
When you hear your friend saying it, your hear it from your friend
Many thanks