To check if you need an apostrophe (') do this:
Example 1
1. The original sentence.
• The teachers class.
2. Now write it with
of.
• The class of the teacher.
3. Looks good.
4. So, we use an apostrophe.
• The teacher’s class.
Example 2
1. The original sentence.
• Ten weeks old.
2. Now write it with of.
• Old of ten weeks.
3. That doesn't work.
4. So, we don’t use an apostrophe.
• Ten weeks old.
When to use apostrophes.