The above has been given as an english home work to correct it's grammar. We think the teacher is wrong by saying that is has 8 grammar mistakes.

Two rivers in York: the river Ouse, and the river Foss.

4 capitalizations
1 colon
1 comma
1 period

There are two rivers in York: the Ouse river and the Foss river.

3 missing capital letters
1 missing opening words (There are) to make it a real sentence.
2 sets of swapped words (river Ouse - Ouse river, river Foss - Foss river)
1 missing colon (could also be a comma)
1 missing period.

That's eight, by my count!

Hope this helps!

If it's supposed to be a complete sentence:

Two rivers are in York, the River Ouse and the River Foss.

Six capital letters, one new word. one comma, one period. That's nine.

If it's just a phrase:

Two rivers in York, the River Ouse and the River Foss, ...

Six capitals, two commas. That's eight.