I think the difference is still maintained by most users:

definite = certain, clear, precise, unlikely to be changed

definitive = something that provides a firm conclusion that cannot be challenged

• In 1993 he wrote a definitive work on the behaviour of stem cells.
• Barry and Susan have now got a definite date for their wedding.

I notice, by the way, that teenagers in Britain these days always appear to prefer definitely to certainly in the following sort of exchange:

• Are you coming to the concert on Saturday? ~ Definitely!

If any of the following pairs of words are easily confused, you might try this sort of activity as a class exercise with a more advanced group of students.

cook and cooker

One is the person who cooks and the other is the stove that food is cooked on. But which is which...?

• He was a really good cook and his spaghetti made me think I was in Italy.
• The cooker was really dirty and I could see that it hadn't been cleaned for weeks.
  

dessert and desert

One is the sweet food that is served at the end of a meal. The other is an area of land where nothing grows and there is very little water. But which is which...?

• For dessert I had chocolate cake with whipped cream and then a bowl of cherries.
• The hot desert sand cut into our faces and we had to close our eyes.

satisfactory and satisfying

One of them describes something that gives you a feeling of fulfilment. The other describes something that it good enough to be acceptable. But which is which...?

• The doctor said he was making satisfactory progress but it seemed very slow to me.
• There's nothing more satisfying than concluding an agreement after five days of talks.

alternate and alternative

One describes something that you can choose to have or do instead of something else. The other describes an activity that is off then on, then off then on again. But which is which...?

• We could see our father only on alternate weekends. Unfortunately not every weekend.
• There is no alternative to a prison sentence for such a serious crime.

principle and principal

One of them describes a general rule or set of beliefs that you try to adhere to. The other means first in order of importance or the person in charge of a school. But which is which...?

• He was a man of very few principles who later came to regret the path his life had taken.
• His principal interest in life was to look after the welfare of others.
  

There are many other pairs that can be used, depending on the level of the class:

• electrical ~ electronic
• economical ~ economic
• historical ~ historic
• complement ~ compliment
• personal ~ personnel
• stationery ~ stationary
• emigrate ~ immigrate
• housework ~ homework
• tasty ~ tasteful
• complexity ~ complication